If forced to use Fidelity as a brokerage I would prefer iShares; however, I would prefer to not use Fidelity. I do most of my investing in non-retirement accounts and as a result tax efficiency is a concern. For this, ETFs (by any provider) and Vanguard mutual funds are typically superior to straight index mutual funds. The reason Vanguard mutual funds are unique in this respect is that they hold a patent allowed assets to be comingled between their ETF and mutual fund share classes, so the superior tax efficiency aspects of the ETF can be shared with the mutual fund (and vice versa).

Also, Vanguard as a corporation is owned by its fund shareholders as opposed to external shareholders while Fidelity is owned by a private group. I don't trust any company enough to lock up my money with them and possibly have to pay capital gains tax to switch later, if they decide to raise fees or make the investment not align with shareholders in some respect. As whitepoplar says, they're a profit-seeking entity. Due to the inverted corporate ownership structure of Vanguard this is not a concern. This argument also applies to iShares.

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.