Hi Tommybhoy its not at all a stupid questions. your best bet is to check out the specs on the O2 website, (here is a link to the comparasons page
http://www.my-xda.com/comp.html) and you can see the 2i and the 2s together. Personally i have an 2s and its been the best thing i have bought.
I think the main differences are that the 2i is slightly faster than the 2i and a slightly better camera, however (please dont quote me on this) im not sure if it has a qwerty keyboard! so that again could be a big difference to you.