Even with index funds there are a few differences.
Expense ratio: This is a big one. However, if they are only off by 0.01 or 0.02% then the other differences start to matter.
Turnover rate: You are correct that this will have the biggest impact if cap gains are distributed in a taxable account. You are also right that this is more likely with the Fidelity fund than the Vanguard fund. VTSAX hasn't had a cap gains distribution in almost 20 years.
Securities lending income: Funds can loan out shares of stock and earn interest income. Some companies pocket this money and other's give it back to share holders. I can't speak for Fidelity, but Vanguard operates at cost so they give this money back. The result is that some of Vanguard's funds pay back more money in securities lending income than they charge in expense ratios. In effect, they pay you to own the shares. VEXAX is a great example. In the case of VTSAX the securities lending income is making up for over 1/3 of the cost of the fund. After you account for this FSKAX and VSTAX are pretty equal in cost.
Tracking: How well does the fund track it's index? If a fund cost 0.02% but it misses it's index by 0.05% then it really isn't as good as a fund that costs 0.03% and perfectly matches it's index.
What do I do to account for all of this? Look at the fund's 10 year returns VS it's benchmark, or in this case since they track the same index we can compare them to each other.
We don't have 10 year numbers on FSKAK, but the 5 year is 10.32% VS VTSAX's 10.33%. VTSAX wins over 1 and 3 year periods as well. This is likely a combination of better index tracking and returning securities lending income back to investors. However, 0.01% over 5 years is pretty small. If it's an IRA I wouldn't worry too much between these two. In a taxable account I would want VTI, the ETF version of VTSAX.
Side note, VTI, the ETF version of VTSAX, is slightly cheaper than VTSAX right now and you can probably still buy VTI at Fidelity. ;-)