I'm going to join the vocal minority here and say I do like SC4 more than Rondo. Maybe I'm just doing something wrong, but I could never seem to get past the 2nd stage of Rondo. It seems like it's just a touch too hard, and I get sick of playing the same level again and again.
In SC4 I've almost gotten to the end, and I really enjoy it. The atmosphere, the music, (same guy who did Axelay and then vanished off the face of the Earth) the experience as a whole, really.
That's not to say I don't like Rondo. What little I've experienced so far I have greatly enjoyed, and I acknowledge that I probably need to just give it another good try. But yeah, at the present, I'd rank SC4 above Rondo.