Well i played the BF2142 beta and demo, it was ok but it just felt too i don't know confused, so many different unlocks, i had to unlock grenades etc.
BF2 i have played since release and i still play it to this day, its quite a lot of fun i must say, but when it goes bad as with 2142 you want to cry...a LOT they can both be very frustrating.
BF2 also has more players.
As for it being a bad sequel its aimed at a slightly different market even though its a pretty much an identical game.
Personally i would go for BF2 every time, they have been out so long you can pick them up for about a tenner just get them both you might as well, or better still try the demos, i know for a fact BF2 has changed quite a bit since the demo but you still get the jist of things.
If you like COD4 chances are you will prefer BF2.