For general use it really doesn't matter I guess. Go with the new one if you need the hard drive space. I chose the new one because I plan to keep it for a while, and I want to store lots of movies.
For less than £100 extra the newer model gets you twice the hard drive space, twice the RAM (DDR3, too) and Mini DisplayPort.. I'd go for that one, doesn't sound like you're going to notice the 9400M in your usage.
If money isn't a worry, I'd go with the new one, as it also has DDR3 memory rather than DDR2 (Faster, and can be upgraded to 8GB rather than just 4GB w/pervious gen iMac).
And you also get more HD space, and the new Mini-Display port.