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Portfolio Composition Difference
VOO is a fund that that tracks the S&P 500 Index, so it primarily holds 500 large cap stocks. Some of the holding will be technically be smaller than large cap (less than 10bn) but their weighting is so small that it doesn't make a material difference.
VTI is a TOTAL US MARKET fund. Rather then only holding 500 large cap stocks, it is holds the entire US market of liquid stocks -- about 4,100 -- more than 8x VOO. This gets exposure to the small and mid-cap markets that VOO lacks.
Morningstar Stock Style Difference
When looking at the mornings star style charts we can see that VTI gets more exposure to the small and mid cap markets with more of a 'value tilt'.
Performance of VTI vs VOO
VTI or VOO: They have both performed very similarly with VTI having a slight out performance. This may be attributed to what is known as the 'size factor' in academia.
Chart created using US Large Cap and US Total Market data, not the actual funds themselves which have not be in existence that long.
VTI or VOO: Which one?
Forget about performance, choosing the right fund in this case is a matter of portfolio composition. If you are going for a simple portfolio like a Boglehead 3 Fund model, using VTI for your entire US market allocation is the way to go. If you are going for a more complex portfolio where you control your weight to difference asset classes such as large cap and small cap, using VOO for large cap exposure is your ticket.