VOO versus VTI is a debate about almost nothing. Good news, actually. Both are Vanguard, both cost 0.03%, both dominated by the same giants.

- VOO: S&P 500, about 500 of the biggest US companies, the cleanest large-cap benchmark - VTI: whole US market, around 4,000 names, small and mid caps tagging along - Overlap is heavy; most of VOO's weight sits inside VTI - The real mistake is owning both and paying for one fund while feeling diversified - Already own one? Do not switch; the tax bill dwarfs the difference

Run the overlap before you add anything: Overlap ($4.99 one-time) checks your portfolio for redundant funds in seconds.