The SSD will do more to make the machine feel snappy overall. However, unless you are doing a lot of disk-bound tasks (processing many photos at once, moving large video files around) or running out of RAM (necessitating virtual memory) most of the benefit will be in things like booting, opening apps, and opening/saving documents. That is, once you're up and running for a given task, the SSD won't make as much difference.
Unfortunately, if you want to play 3D games, the much better GPU in the higher model will be more important. Even the good GPU isn't particularly strong for games running at the native resolution, but the lower-end one will likely prove frustrating for any games of recent vintage. Check out Bare Feats' game benchmarks to get an idea of what frame rates you can expect with each one.
So the trade-off is that the SSD will give you a (much) more responsive system overall, but you'll probably have to play games at a lower (non-native) resolution to have satisfactory performance.
I'm planning to order an iMac later this summer, and for me the better GPU is mandatory. The SSD would be nice, but I'm not sure the cost/benefit is in its favor. There's also the hassle of either a 256GB space constraint if you get just the SSD or the semi-manual management of your files if you get SSD + HD. (Keeping big directories like your iTunes and iPhoto/Aperture/Lightroom libraries on the HD and everything else on the SSD is one way to go, but then you lose the speed benefit of the SSD for photo work. It's probably not important for music.)
EDIT: The CPU difference probably isn't important. Your multitasking isn't going to saturate all four cores, with or without multithreading. The i7's single-thread performance will be better, but not noticeably for most tasks. The only situation where it would make a difference would be long-running, multi-threaded tasks, like rendering 3D models, converting/rendering video, certain scientific/statistical applications IF written to be multi-threaded, etc.
