This years cartels and last years P1s (discontinued for this year) have the same highback (or very similar). I personally like this change for the Cartels as the old skyback was a bit ridiculous (high and a lot of forward lean). I have just replaced my cartels with Targas.
So the biggest difference between the 2 will be the baseplate. The cartels are a 1 piece nylon while the P1s are 2 piece. Cartel is a tad bit stiffer. Depends what you are after really...
Personally, if the Cartels are pricer and soldout...just get the P1s?...unless of course you want this years tech like cantbeds..