Fidelity's Total Stock Market Index Fund, FSKAX, which I take it is not available to you, tracks the Dow Jones U.S. Total Stock Market Index. (It's from Dow Jones but it is a normal, cap-weighted, total market index, with no connection to the Dow Jones Industrial Average
FXAIX tracks the S&P 500 Index.
FSMAX tracks the Dow Jones US Completion Stock Market Index.
The Dow Jones US Completion Total Stock Market Index
a subindex of the Dow Jones U.S. Total Stock Market Index that excludes components of the S&P 500®
According to S&P
The index includes 500 leading companies and covers approximately 80% of available market capitalization.
The 80% number hasn't changed much in all the time I've been paying attention.
Notice, by the way, the careful phrasing: "500 leading companies," not
"the 500 largest companies by market capitalization."
As others have noted, the S&P 500 and the total stock market are actually surprisingly similar, because not only does the S&P 500 include 80% of the market by cap weight (i.e. by dollar value), but the remaining 20% doesn't really behave all that differently from the rest--the correlation
between the S&P 500 index and the completion index has been about 0.93. As a result, the overall financial behavior has been really very similar:
Personally, yes, I would go with 80% FXAIX and 20% FSMAX if they are both conveniently available in your 401(k) plan, and the biggest single reason is that you do not have to waste time reading overwrought nonsense about what it means for the S&P 500 when/if Tesla will be added to the S&P 500, because it is in FSMAX already, and if when/if Tesla is added to FXAIX it will be removed from FSMAX.
But someone who really prizes simplicity for the sake of simplicity could just shrug and go with FXAIX.