I think a lot depends on your expected flying patterns in 2007. If you'll be doing a lot of flying on AC, then I'd go for SE. There is a lot of talk about instant KK being the best benefit and it is a good one, but I actually find that the best benefit of being SE is that you are ahead of all the E's for upgrades (sorry E's).

If a good chunk of your flying will be on one of these other carriers, then there may be value in getting status with them too.