It really depends on what you mean by "Better" -Better Graphics? -Better Story line? -Better voice acting? -Better cut scene footage?
If i were to base my answer off these qualities I would have to pass my vote for the original Sonny. I felt that It set up a great story line for the rest of the series to follow through, the graphics i felt both games were evenly qualified, and for voice acting I felt Sonny 1 had a bit more emotional appeal that helped the characters develop into what they are now in Sonny 2. However I will give that Sonny 2 did have a bit more action and a few better cut scenes than the original sonny did in my own personal opinion.
But you people leave out the little things that really make a difference. For Sonny 2 all the classes have different abilities. In Sonny to there are 7 zones, and all of them were much longer then the ones in Sonny 2. Sonny 1 has 32 levels and Sonny 2 has 65 ( including Zone 6 & 7). Sonny 1 didn't develop Veradaux as a humorous person like they did in Sonny 2. But as for ffa's post. None of those things involved game play, or strategy, or difficulty setting. Sonny 2 had much more replay value then Sonny 1 could ever offer. So after this whole rant you only expect me to say Sonny 2.