I've got 3 bibs - Assos, Performane, and PI, and one pair of Performance elite shorts.

I prefer the shorts. The bibs are fine as well, but the waiststrap never bothers me, and the small convenience of not having shoulder straps means I can adjust my HRM and go to bathroom easier. My shorts have never been a problem shifting. Bibs actually feel a bit weird to me without the waiststrap as well.