335i:
Pro's: 306bhp, dual exit exhuasts at the back, 0-60 5.5 seconds (Tho in reality they are quicker than the stats suggest).
Cons: Based on older M54 engine, HPFP is a significant weak point, not particularly economical at 31mpg combined, not quite as crisp throttle response.
330i:
Pros: 272bhp, 0-60 5.9 seconds but with a combined mpg figure of 39 - it's allegedly significantly more economical than a 335i without being hugely down on power. Brand new N53 engine. Crisper throttle response. Cheaper so you get a newer car for the money.
Cons: Doesn't quite have the midrange thurst of a 335i. Not as easy to find. Isn't 'the top one'.
In reality both are fast cars, neither will leave you wanting power wise, both are far quicker than I'd ever need anyway. What the 330i lacks in power it makes up for in real world usabilty - ie 40mpg on a run is better than 30mpg and cuts the cost of those 1000 mile at a time jaunts