According to the Intel spec sheet, the turbo of the i5 is 2.7 ghz, which is fairly significant from a base clock of 1.8. How often it happens depends on the programs you run.
If they're the same price, you might as well get the i5 since it is superior to the i3, there is nothing that it is inferior in.
I can tell you my i7 in my ultra book runs with the turbo on most of the time at 2.5ghz, I would spring for the i5, but I wouldn't go i7 for an ultrabook (mine just happened to have i7 for the same price but I doubt the extra cache on the i7 helps all that much).