Anyway, Gore Windstopper and Polartec Windbloc both operate with the same principle: They sandwich a very thin windproof material (Gore uses a light version of Gore-Tex; Polartec a urethane membrane) into the fleece or pile. That's a somewhat complicated manufacturing process, hence the relatively high price of these garments (Cabela's offers a nice-looking Windstopper fleece jacket for $140, which ain't bad). Polartec Windbloc-ACT, on the other hand, tries to make the fleece itself as windproof as possible. Polartec claims it blocks 98 percent of the wind, so you do get a little extra ventilation. It's also cheaper to make, so a company such as L.L. Bean can sell its ACT-based Microburst Fleece Jacket for a very reasonable $125. Personally, I'd probably get one of those before I forked over the extra $100 for the R4, although it is a very nice piece.
In either case, I don't see any harm at all in draping a windshell over such a fleece for some extra moisture protection, although the wind-blocking fleeces are also surprisingly mist resistant.