The only difference I know of is the impellers, the old OE pumps used a cast iron impeller, which is said to move a bit more coolant than the stamped steel impellers, but I decided on going with stamped steel, because I think the cast iron ones have a higher chance at corroding, not that they are known to do so.
I am not a water pump expert, but I honestly think if you run a stock motor, you change your coolant out when you are supposed to, and have no HP adding upgrades, then the type of impeller should not really matter.
My old water pump made it to 135k, and I expect the new one to last just as long.
I went with the more expensive AC Delco WP, it comes with the aluminum framed gasket, what I find odd though is that is says " Stamped Steel Impeller GM Original Equipment", I could have sworn OE design was cast iron.
If money is tight you could always go with the cheaper one, as long as you service your coolant system regularly, then I don't see why it would not last, and while I am not totally positive, I don't see why you couldn't use the aluminum framed gasket with the cheaper pump.
If you want to learn more you can check out some of the links via this search, I did, and to me it sounds like it does not really matter what type of impeller the WP has.