IS or L glass on tele zoom

With £400 to £500 to play with, I see I can get either the EF 70-300mm f/4.0-5.6 IS USM Lens or for a little more the EF 70-200mm f/4.0 L USM.

So, my question to the assembled masses... which would you go for. I'm thinking the IS will more then outweigh the f/5.6 end of the smaller lens. But for a semi-serious hobbyist, is the L series glass worth the extra £££ (or $$$) and losing the extra 100mm of zoom

Will be used with a 400D and unlikely to upgrade to a "two digit" body.