Replying to What zoom lens?

Having problems deciding

At the top end of my budget is:

Canon EF 100-300mm f/4.5-5.6 USM @ £285
Canon EF 75-300mm f4-5.6 USM III Lens @ £235
Those are the two options I am currently considering. I am wondering whether it's worth the extra money for the first lens with the snappier ring USM and probably slightly better optics over all or whether I should save my £££ and get the second (plus the advantage of the wider focal range and f/0.5 extra).

Tough decision my fellow NSers! What do you think?