I was able to do a back to back comparison of my 11m Sonic and 2013 19m Edge (used to be mine) today in 8-10 knots of wind (measured 200' above water, 2 miles offshore). I was able to switch kites very quickly, so it was a good back to back comparison riding the two kites in the same wind, on the same board. I'm 165 lbs on Spotz2.
The most noticeable difference was the amount of power the kites made in the loop. Hands down, the 11m Sonic made way more power when you looped it to get going than the 19m Edge did. Because of this it was easier to get going on the 11m Sonic than the 19m Edge. I knew size for size, the foil kites made more pull in the loop, but I was really surprised that the 11m Sonic had more pull than the 19m Edge when you moved it.
The other surprising thing was how stable the Edge was. I didn't drop either kite, but the 11m Sonic was more nervous in the air when I wasn't moving, needed some attention. Usually in this wind, I'm on my 15m Sonic which hangs in the air a little better (better weight/projected area) so I'll be interested to try the same back to back with the 19m Edge and 15m Sonic.
Other areas:
Upwind angle: Sonic pointed noticeably higher
Board speed: Sonic was noticeably faster
Small jumps: A little higher on Edge, a little more hang time on Sonic
Float through foiling tacks: Slightly more float on Sonic, but Edge floated pretty well
Turning: Edge turns tighter. You can stall the Sonic and get it to spin around in place, but generally it does very wide sweeping turns with lots of power. Edge had less power when you moved it, but it turns tighter and faster.
Relaunch: The foil kites relaunch in almost no wind as long as they dont get tangled on the way down.
Swim in: Didn't have to today, but Edge 19m is WAY nicer to swim in with
There are tradeoffs, but for me, the race foil kites are a game changer. Blasting around at high speed in 7-8 knots is great. I'll find the occasional swim in worth it for the added performance.