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.