To add a (very) little to F3G's post, in my experience there is little to tell the -151 and -161 apart (at least, in the early stages of training - effects of controls and circuits) whereas the extra horsepower of the -181 (the Archer-II) is noticable even to a beginner like myself. I'm planning on grabbing the Archer for Nav work - at my club there is no difference in price, but YMMV.
Oh, and I'm feeling a little sheepish for forgetting the Arrow, which as others have pointed out is a different beast. I'm firmly in
ab initio spamcan mode
