Alright, played both demos now.
First of all PES. The second demo is very different to the first and I actually like it. The ball and dribbling animations look a lot better and it's got the old PES feel to it. Think I'm going to buy it this year.
As for the FIFA demo, it's left me feeling very meh. Apart from the new defending and collision system, it feels like exactly the same game. I love FIFA 11 though, so that isn't a totally bad thing and I remember I was very unimpressed with the FIFA 10 and 11 demos and the final game turned out to be much better, so hopefully that'll be the case here.
So yeah, I'll be buying both.