I can't speak to the 529 directly, but someone pointed out to me that Vanguard's money market pays better than Fidelity's. One year 4.77% vs. 4.53%. I have my Roth and Taxable Brokerage with Vanguard, my work 403b and HSA with Fidelity. If it is a toss up, I prefer Vanguard because while both have low fees, I feel like Fidelity's are more of a result of having to compete with Vanguard rather than for altruistic purposes. Fidelity is privately owned, Vanguard is owned by it's funds (and therefore its investors).
If I had everything else at Fidelity, I'd probably go with Fidelity. Keep it simple.