SPI belt, Amphipod, and FlipBelt all are good but differ. SPI belt and Amphipod have adjustable cinches to tighten how close they hold to your body, so they don't bounce. FlipBelt is sold in different waist sizes. FlipBelt and Amphipod have wide waistbands so they don't roll as much as the SPI belt does (manufacturer says it doesn't roll, but not true). SPI does have an "Endurance" model with a wider band, PLUS that model has loops for holding race bib, PLUS an extra elastic band for 6 gels on the outside and room for 8 on the insize. The normal SPI doesn't hold that much, maybe a cell phone and your pump and sunglasses if you don't mind possibly breaking the glasses. Amphipod looks orthopedic. Right now my experience is that the Amphipod bounces less and holds more, but I haven't tried teh SPI Endurance.
FlipBelt goes all the way round your waist, so it's easiest to adjust placement of load.
I've been T1 since 1942 and pumping since 1995. I have had 4 different pumps, and I have worn each of them clipped to the waist band of my bike shorts, with 100 mm tubing tucked inside the shorts. I have never felt the need to have a belt for the pump.