Not parent, but I wouldn't use these funds because there's a fundamental conflict of interest. Fidelity is a private company that seeks to profit from its customers. Vanguard is a company owned by its customers.

What happens in the future if passive indexing falls out of favor, there's an exodus out of index funds, and Fidelity jacks up fees? If you're investing in a taxable account, you'd (presumably) incur a capital gain to leave the fund. At Vanguard, expenses may increase, but they won't seek to profit off of you. In a tax-advantaged account, I suppose the Fidelity funds are fine.

I'm curious to hear tanderson92's answer, though.