How to Compare ETFs

Before any metric, answer one question: what does the fund actually own, and what index or strategy does it follow. This is what tells you whether two funds are the same bet or different ones. Two S&P 500 funds hold the same roughly 500 companies and will move almost identically; an S&P 500 fund and a Nasdaq-100 fund hold different things and are not substitutes. Read the index and the top holdings first, because every other metric is secondary to whether the funds are even comparable.

The practical version of this is overlap, covered below: even funds with different names often hold the same mega-caps near the top, so they are more similar than they look. Start with holdings, and the rest of the comparison has context.

The expense ratio is the annual fee a fund charges, expressed as a percentage of assets. It looks tiny, but it compounds: on a long-held position the gap between 0.03% and 0.20% works out to a meaningful sum over decades, because the fee is paid every year on the full balance. For a fund tracking a mainstream index, you rarely need to pay more than a few basis points, since a cheaper fund on the same index keeps more of the return.

The expense ratio matters most for long-term core holdings and least for short trades. It is the first number to check when two funds track the same index, because it is often the clearest difference between them.

Holdings tell you what the fund is; overlap tells you what a new fund adds to your portfolio. The two are different questions. A fund can be excellent on its own and still be a poor addition if it stacks weight on names you already hold. Broad US funds (an S&P 500 fund, a total-market fund, a large-cap growth fund, a Nasdaq-100 fund) all hold the same handful of mega-caps near the top, so combining several of them concentrates those names rather than diversifying.

Checking overlap before adding a fund is one of the most useful things you can do, because overlap is the most common reason a portfolio is less diversified than it appears. Comparing top holdings and sector weights is the manual version; a connected app can quantify it across everything you own at once.

AUM is the total money in the fund, and it is a rough proxy for liquidity. Larger funds tend to trade with tighter spreads and deeper order books, so orders fill at better prices, which matters most for large trades. Very small funds carry a second risk: they can close, which forces a sale that may be taxable and is rarely on your schedule. AUM is not a quality score, but a very small fund deserves a second look before it becomes a long-term holding.

The bid-ask spread is the gap between the highest price a buyer will pay and the lowest a seller will accept. You effectively pay about half that spread each time you trade, so it is a real, if hidden, cost. Large, heavily traded funds have very tight spreads measured in pennies; small or niche funds can have wide ones. The spread matters most for active traders and for large orders, and least for someone making small, infrequent purchases in a broad fund.

For a passive fund, the job is to match its index, and tracking error measures how well it does that. A fund whose return sits almost exactly on its index has low tracking error, which is what you want. Persistent or large tracking error means the fund is drifting from the thing it claims to replicate, through fees, sampling (holding a representative slice rather than every name), or trading frictions. When two funds track the same index, lower tracking error is the tie-breaker after cost.

In a taxable account, what you keep after taxes can differ from the headline return. Index ETFs are generally tax efficient because their in-kind creation and redemption mechanism lets them avoid passing through many capital gains, so you mostly owe tax only when you sell. Tax efficiency varies, though: funds with high turnover, active strategies, or options overlays (such as some covered-call income funds) distribute more along the way. In a tax-advantaged account like an IRA or 401(k), the difference matters far less, which is why some investors place their least tax-efficient funds there.

Yield is the income a fund pays as a percentage of its price, and it is one input rather than the goal. A higher yield can reflect genuinely higher-income holdings, but it can also come from a falling price or a strategy that trades growth for income. For total return, the combination of price appreciation and dividends matters more than the headline yield, so a very high number is a prompt to ask why rather than a reason to buy.

Take VOO and SPY. Both track the S&P 500, so the first two checks, what they hold and tracking error, come back nearly identical: same roughly 500 companies, both following the index closely. That tells you this is the same bet, so the comparison narrows to the remaining fields.

On cost, VOO charges around 0.03% versus SPY's 0.0945%, an edge to VOO that compounds for a long-term holder. On liquidity and bid-ask spread, SPY has the deepest options market and tightest spreads of any ETF, an edge for active traders and large orders. Both are large and tax-efficient index funds. So the honest conclusion is not that one is better, but that cost-focused buy-and-hold leans VOO while trading and options strategies lean SPY. That is what a side-by-side comparison is for: it turns “which is best” into “which fits how I will use it”.

No single number decides it. For a long-term core, weight the expense ratio and tax efficiency; for active trading, weight liquidity and the spread; for deciding whether to add a fund at all, weight overlap with what you already own.

Most of these fields are public, but two are hard to do by hand: overlap with what you already own, and how a fund has actually performed against a benchmark over the window you care about. Both depend on your real holdings, which is where an AI assistant connected to your account helps more than a generic screener. You can ask, in plain language, whether a fund overlaps with what you hold, which of two funds fits a given slot, and how each has done against the S&P 500.

That is what Walnut does. It connects your existing brokerage through SnapTrade and lets you compare funds through Claude, ChatGPT, or a built-in assistant, see overlap across everything you own, and track each position against the market. It is read-only by default, and you approve any trade. Walnut is not an investment adviser; it helps you compare and act on your own portfolio rather than telling you what to buy.

Comparing ETFs well is a short checklist, not a single ranking. Start with what each fund holds and the index it tracks, because that decides whether they are even the same bet. Then weigh the expense ratio, overlap, AUM and liquidity, bid-ask spread, tracking error, tax efficiency, and yield, leaning on the ones that match how you will use the fund. When two funds track the same index, cost and liquidity usually decide it; when they track different indexes, the holdings do.

To go deeper on specific funds, browse the per-fund pages under ETF guides, see the best fund in each category in the best ETF in every category, or read the goal-grouped roundup in best ETFs to invest in for 2026.