I dont have any experience with either. But I understand as performance is concerned, theres not a whole lot of difference. And that GX50 looks big as heII. Id go with a GX35 unless you're morbidly obese, bike up mountains or something.
It's 3-1/2 inches taller... which makes it seem much bigger.
Note that Honda recently 'derated' the HP for the GX25, 35 & 50. (They changed units, from BHP to 'Net HP.') I don't know whether the HP difference is real, or whether there's just a difference in KW between a Brake HP and a Net HP....
The key difference between the two engines is this: The GX50 has more torque at a lower RPM (GX50:2.2 FtLb@4500, GX35:1.4 FtLb@5500.) Both engines develop maximum HP at 7000 RPM. This means that the GX50 will pull better when the engine slows down - when going up hills, or you're accelerating.
But, nothing comes for free... 5 more pounds, plus, the engine weight is centered higher on the bike. In addition, while you may be legal in CA, in many states, 48cc's (or more) mean that the bike has to be registered as a motor vehicle.
I recently went from the friction set-up to the staton chain drive over a nuvinci and the difference is exponential!!!
With the friction i HAD to pedal assist the bike to get about 7 mph uphil. after the addition of the nuvinci i can easily climb most all hills and that same hill i struggled with i now cruise along at 15 mph with no assist.
top speed between the 2 for me is not different either way. i can just apply the power down low much better with the nuvinci hub and acceleration is much better. top speed averages 30 mph.
The GX50 if the tank and fly wheel shroud are off it doesnt look very big. At 210 lbs and experience riding with the 50cc I couldnt imagine any thing smaller with less power. The 50cc is the perfect Mab motor IMHO.