What is the difference between an impact driver and an impact wrench?
Most driver bit sets come with a 1/4" square drive adapter (my small Milwaukee set also came with a 3/8" drive as well, though I don't know if that's common).  With that one bit, your driver is instantly almost exactly the same as a 1/4" impact wrench.  You'd have to look at torque specs to see if the models you're considering are comparable, but I bet they are.  Those little impact drivers have come a long way -- battery life is great and the torque is hard to believe.

I'd only consider a square drive impact wrench for heavier automotive (or other mechanical) use, and only in a larger size like 1/2" so it can provide some real help in removing tough fasteners.