NW is nice but it doesn't go where you want it to go. I'd get in *A and go for LH M&M, especially if you're outside Germany/Austria/Poland. LH and Varig have flights that are non-stop between Brazil/Argentina and Europe. You'd have a hard time with good flights on the SkyTeam alliance for that routing.

I like NW service (I just started flying them) and US/UA as well, but I'm not all that picky. I'd really pick your new program by finding the alliance that works the best for you, then pick the program within that alliance that seems the best for your flights (and whose planes you will be on the most often). For you I'd guess *A, and LH M&M inside that alliance.