I think Splosion Man is a longer game, however I think Sonic 4 is a much more visually varied game.
Splosion Man is essentially the same environment with 3-4 different color variations and around 15 "acts" per color variation. Sonic 4 has 5-6 environments (first 4 zones plus final zone plus bonus stage) with 4 acts per zone for the first four, one act for the 5th one and 7 for the special stage. About 24 total.
So while on paper Splosion Man has more individual stages, Sonic 4 beats it visually by offering moe varied environments. Heck, even some individual acts within a zone are different from one another. I'm loving Splosion Man myself, but have found that the environments lose their appeal about halfway through. Same cross section mechanical labs with molten rock inbetween. Sonic, however short, gives a grab bag of worlds to run through.
Now Comic Jumper, that looks to beat both Splosion Man AND Sonic 4 in the varied environments department.
btw Sharky, I'd love to be on the inevitable SEGAbits podcast discussing Sonic 4 when it releases. You wouldn't want it to just be you alone getting beaten up by the haters, would you? (besides, its looking like we'll be the only members to actually buy it)