I think it makes sense to have account at both if you buy ishares ETF and Vanguard ETF shares. Buy ishares ETF at Fidelity account and buy Vanguard ETF at Vanguard. They are free if you do.
$4.95 per trade for us buy & hold types is so inconsequential that, IMHO, it should not be considered in the decision. My twice-a-year trip to Starbucks is a bigger financial commitment.