Both good examples of Mini's.
The GP1 features a GP specifc body kit, interior, wheels, intercooler, suspension etc
The GP2 is a JCW with some stickers
Due to the above the GP1 feels a more special car and it was made in a lot lower numbers.
It depends if you are going to modify the car as well and what you want from it. Its very easy and cheap to get a GP2 to 280+bhp with remap, intercooler and exhaust.
A GP1 is cheap to get to 230-240bhp. After that it gets very expensive to get anywhere near 280bhp with the need for performance heads etc.
The GP2 as you say is cheaper to run as the GP1 is relatively thirsty and high on tax.
For me I am addicted to the supercharger whine, I have owned all three generations of the new mini and I currently own a 1st gen supercharged R53 Cooper S and the 2014 3rd gen 2.0 turbo F56 Cooper S, which I have mapped to 300bhp
The 2nd gen for me with its Pug engine is a worry, however the GP2 from memory does have the N18 engine which is stronger than the N14. Both pug engines however the N14 was revised to the N18 to try and solve some of the reliability issues. I always worried with my 2nd gen Clubman JCW(N14) was going to go bang at any point, I dont have that same fear with the other two I now own. This shouldn't worry you so much as the N18 is stronger, but its why I didn't consider a 2nd gen when building a track car.