The GenII is more tunable, you can do more with the throttle and brake curves although TBH I never needed half the settings on my GT12, the Gen1 itself is very tunable and IMO has everything you need.
As discussed I personally found it very difficult to solder up the GenII power wires.
The Gen1 still has solder posts but despite my best efforts at ham-fisted soldering I never managed to remove any of them from the PCB so I think on the Gen1 the posts must be attached to the PCB using a solder with a higher melting point than the stuff available to the public.
This makes life a lot easier, like I say I once replaced one in about 20 mins between heats
