I would suggest going for the Core2Duo for a couple of reasons:
1) Its effectively rev-B hardware and has improved on the problems inherent in the original Core Duo release (most notably in terms of heat - the new ones run a *lot* cooler)
2) Can easily upgrade the RAM later if your budget won't stretch to it (I've just bought a macbook C2D, stuck all the apps I want on it and its still bloody quick - I'd originally planned on adding an extra gig but for the time being I'm not going to bother - as it stands I don't need it).
Or to be more brief - Core2Duo for all the reasons suggested by the above posts