I'm biased since MM3 is the only MM game I owned growing up, and I played the hell out of it.The slide was a nice touch, but the true reason 3 is better than 2 is that there's much better weapon balance. Metal Man's weapon is just way too overpowered.
That, and the fact that 3 has noting as rage-inducing as that Crash Bomb-only boss in Wily's Castle.
I honestly prefer it because I enjoy the addition of the slide, I find the soundtrack to be better/catchier, I like the weapons and Robot Masters themselves more, and I'm keen on new characters such as Rush and Proto Man.