hi ray,
there are a few notable differences between the 3rd and 4th gen glocks, most notably the recoil system. the 4th gen have new double springs which are supposed to reduce the flip of the .40 cal. i haven't shot one though.
they also have an interchangeable backstrap, which i am not particularly fond of (in general). but in and of itself, it wouldn't prevent me from owning a 4th gen.
the new mags also have cutouts on both sides so the mag release can be switched to lefty. this should not affect the use of any mags in either gen.
the grip texturing has changed as well, it looks to be less aggressive.
the only real drawback i see is the recoil springs are not interchangeable, so the armory (or you) would have to purchase the newer ones.
unless you are really set on a gen 3, i see no reason not to take the deal on the new model.
hope that helps,
mark