As a hobby, I buy dropped bikes in the fall/winter from auto auctions and rebuild them before the snow melts for the next riding season. Last winter, I repaired/restored four 650s and two 300s. I have some comments to offer based on both riding and repair experience.
Riding-wise, I picked up a '13 650 at auction last summer, fixed 'er up, and put about 3000 miles on it. I thought it was decent enough, but I missed the adjustable suspension of my previous ZZR 600. I figured I would fix and drive one of the '15 ABS 650 models for this season. However, after fixing and riding all the bikes, I kept a '14 300 SE. Of the various bikes I've had the past three summers, it is by far the easiest and most enjoyable to ride. The hands-down winner.
You'd think a 650 would ride as good or better as a 300 but, alas, not so. As others have previously stated in this thread, the 650 suspension is horrible, particularly the front forks. Well, the rear is pretty bad too. On our lousy Michigan roads, the 650 pogo sticks all over the place. Encounter bumps at an intersection and it's downright dangerous, IMO. Compared to the tossable 300, the 650 is an overweight, under-engineered and dated design. The 650 engine is nice, but wind it out and you're doing 100 MPH. As any 300 rider knows, you can wind it out, have a blast, but stay within a reasonably legal speed. Well, almost. . . The 300 suspension isn't adjustable either but it's remarkably smooth by comparison.
As for build quality, again the 300 trumps the 650. The 300 snaps together better and is more robust in some key areas, like the headlight mount structure for instance. On the 300, it's a welded, tubular cage. On the 650, it's a flimsy bracket bolted to the steering tube. No comparison on strength and rigidity. I could go on and on about the engineering differences, it becomes obvious when you have to piece together all the plastic bits on a bike.
Add in nice features like the slipper clutch, smoother throttle operation (a big deal if you do a lot of slower, city style driving) and the 300 shines. Finally, after an hour of driving each bike, my forearms are tingling with the 650 from some unabated handlebar buzzing. The 300? Nothing. Could ride it all day.