The i3 would be the better CPU for gaming. Its newer and faster technology than the other two chips you mentioned and it also has hyperthreading so it appears to have 4 cores giving it better multi-tasking performance.
Are you upgrading or going for a new build? If so you should definitely go for the i3 since the LGA775 socket is now dead. If you're on a budget have you considered an AMD chip? They offer very good performance at great prices. You could probably get a chip that will outperform the i3 for about the same price.