I use the belt system. It's much more convenient and in the long run more comfortable than carrying a shoulder bag.
I've shot two events on a similar setup.
1. Founder's Day (afternoon - dinner)
Carrying a Domke J1, loaded with a 580EX, Lightsphere, Sto-fen, 17-50 f/2.8, 70-200 f/2.8 IS USM and 30D. When shooting, most of the time the 17-50 is on the camera. This event left my right shoulder aching for a good two days.
2. Christmas Musical (3hr event)
Carrying Pro-Speed Belt System. Transported to event location via Airport Antidote. Shot on a 30D, 17-50 f/2.8, 70-200 f/2.8, 35mm f/2, Sigma 10-20mm f/4-5.6, 580EX with Lightsphere. Flash usage at reception and meet-the-actors after the musical.
I felt more tired after 1., well, cos it's a longer event, but the ergonomics of the belt system reach out to me more. It's easier to get to everything. No need to lift the flap off your bag and dig around. The only issue I have is it's a little hard sitting in regular chairs when you're so "fat". Worse still are those with armrests.