I have both. They both breathe beautifully. Both are small and light.
B2: Piston design 1st stage longer and thinner with all LP hose routing coming off swivel at bottom. Inline swivel and first stage swivel make second stage weightless. 1st stage not environmentally sealed.
F7: Diaphragm design 1st stage shorter and wider with a wide variety of hose routing options. Miflex hose makes second stage almost weightless. 1st stage environmentally sealed.
For moderate to warm water, the B2 is probably a bit better because it eliminates jaw fatigue and is tried and true, but overall I prefer the F7 because much of my diving is cold water and my options for routing drysuit inflator hose, octo hose, primary second stage hose, LPI hose and HP hoses/transmitters are more
ergonomic and flexible. Plus stylistically, the F7 is unique.