why would you want to compare it anyway? Its not even finished. Not to mention that i highly doubt that there are any "free" simulations out there and if so then i believe that the quality cant be that good to be even considerable for a comparison.
Well, if you have a PS3 and its connected to the network, I guess you can compare GT:5P to it (since both are incomplete I guess). Mainly:
LFS >GT:5P in terms of physics and the related.
GT:5P <LFS in graphics.
Personally I'd go for LFS because it feels better than GT5P. I like a game that plays well, not look like it plays well. I mean GT5P isn't THAT bad but it feels "dead" when you compare it to LFS. The cars in GT5P don't feel as lifelike. They feel more numb and restricted by whatever the programmers did to it.