Same money, same gross return, two expense ratios: see what the wrapper costs over time.
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Each product compounds at its own net rate - gross return minus expense ratio - once a year. Ending balance is the starting amount times (1 + (gross return - expense ratio) divided by 100) raised to the number of years. The no-fee column grows the same amount at the full gross return, so each wrapper's drag reads separately, not just the gap between them. Funds accrue the ratio daily, so the true drag is a shade smaller.
Defaults: $100,000, 6% gross, 20 years, a 0.05% ETF against a 0.75% fund. The ETF ends at $317,701.47 and the fund at $278,254.43, so the ETF is $39,447.04 ahead. Against the $320,713.55 no-fee benchmark the cheap wrapper gave up $3,012.08 and the expensive one $42,459.12.
Buying. An ETF trades on an exchange at a market price all session: you can cross a bid/ask spread and the fill can differ from net asset value. A mutual fund transacts once a day, at the NAV computed after the close, so an order before the cutoff prices at a NAV you do not know when you click. An ETF's minimum is one share, or a fraction on some brokers; many funds state minimums, commonly a few thousand dollars at large US families, far more in institutional classes.
Taxes. ETFs are usually more tax-efficient in a taxable account because authorized participants create and redeem shares in kind - swapping securities instead of forcing the fund to sell - so embedded gains can leave without being realised across shareholders. That makes the wrapper tidier, not the investment better, and in an IRA or 401(k) it is mostly beside the point.
The fund can be cheaper. At larger balances an index mutual fund can cost less than the matching ETF: its minimums buy a cheaper share class and there is no spread to cross.
Outside the ratio. Sales loads - front-end, back-end, contingent deferred - are not in an expense ratio. A 12b-1 fee usually is, but is easy to miss in a headline number, and the class you can buy may cost more than the advertised one. Spreads and commissions are not in a ratio either: read the prospectus fee table.
Nothing about tax, which often decides a taxable account and can outweigh a full expense ratio; nothing about tracking error or securities lending; nothing about liquidity, since a thinly traded ETF can be quoted wide. A ratio also says nothing about whether the holdings fit what you own.
Browser-only arithmetic; nothing you type leaves the page. No wrapper is recommended: a lower ratio is one input among several, and fees buy management, some of which is worth paying for.