This kind of thread always prompts fanboy arguments and C&C3 HAS a lot of fanboys.
My case example being the day that first few minutes of scattered footage of C&C3 was released there were about 10 people in this forum alone already claiming it was better than SupCom. I mean it may look pretty, but Doom3 looked pretty, saying it is better is just totally pretentious.
C&C3 will no doubt offer a better single player campaign than SupCom. Of that I am sure!
However I think the real test for any of these types of games is the last ability. Hate it or love it StarCraft has lasted ALONG time proving itself to be a pretty robust game.
I can see myself wanting to play skirmishes and multiplayer battles in C&C3, but I honestly got bored of playing the demo real quick. It just doesn't offer the complexity and unit diversity of SupCom.
C&C3 has ALWAYS been about quick tank rushes and super weapons. It is great fun to play with mates around a LAN because it isn't something that requires much thought to play!
I feel SupCom offers a lot more diversity and some seriously new evolutions in gameplay. I don't feel let down by the hype of SupCom at all. Where as I have already uninstalled my C&C3 demo.
I will most likely be buying C&C3, but I expect to be still playing CoH and SupCom long after Xmas this year when the NEXT big thing is out.
More importantly to the OP though is that anyone who can afford a rig to run them both can easily afford both games. Each offer different aspects of fun and gameplay, so please don't compare chalk to cheese!