If buying new, the extra £110 to go for a 4 is very much worth it. But then the extra £70 for a 4S is probably worth it too.
If you’re picking up a secondhand one from someone upgrading, the 3GS should do OK – but there’s not many people running ios5 yet on the 3GS to give feedback on how good (or otherwise) it is.
As ever, battery life is very dependant on how you use it. When mine is in “festival mode” (no 3G, no wifi, no bluetooth, no push mail, dim screen, etc) it’ll last a few days easily. With all that stuff on I’m charging it every day, it will normally be about 30-40% by bed.