The P1 has a 160mm bottom rail, the GT1 has an 80mm bottom rail. Both should be completely sufficient for sim racing as a rigid and sturdy base. The GT1 seems to have a more versatile pedal deck, allowing more height and adjustment that the P1 pedal deck. That said, the pedal deck on the P1 seems to look a little nicer, but it will not be as adjustable as the GT1, which may or may not affect you personally. The P1 also comes standard with a brace for the gear shift/accessory mount that connects the end of it with the bottom frame, the GT1 does not and has the arm finish in mid air. I think the seat mounting frame is identical on both, but the GT1 may be a slightly wider chassis overall which may affect the dimensions of the seat mount frame. We're talking 50mm or so, so not a great deal. Both have an option of either a front mounted wheel deck, or a traditional shelf style wheel deck mount and both have the same adjust-ability.
Apart from that, they are very similar cockpits. I chose the GT1 over the P1 mainly because I wanted the added versatility in the pedal deck. It seems you can get your pedals up high enough to simulate lying down in an F1 car, provided you get your seat low enough to maintain that level with the maximum height of the pedal deck. In the end. I actually added the bracing for the side mount to mine as I liked the idea of that arm being fully fixed. I run shifter and handbrake on that arm so it's highly beneficial to have it especially with a handbrake.
I am super happy with mine, it took some messing around to get it exactly where I wanted it to be, but that was mainly due to me adding certain bits not standard on the GT1 build and needing extra profile to mount my V3 motion platform and my seat to said platform, which was a little awkward. Check out my build log with the GT1 for a more in depth look at the chassis in the way I have it configured. It may help you make a decision