I personally prefer the Mk series vs the 22/45. The grip looks pretty odd but I find it more comfortable personally.
Now, as to Mk1, Mk2, and Mk3 - that's a personal choice. The Mk3 is the only one still in production so that's your choice if buying new, but there are plenty of all on the used market.
My opinion:
Mk1 - Good framework, awesome gun to shoot.
Mk2 - They made some genuine improvements to the design. Extra round in the magazine, bolt hold-open on empty.
Mk3 - Lawyerized version. Nothing of value added over the Mk2.
I personally have a Standard model which is the Mk1 pattern (even though not labeled as such). I love shooting it - the only thing I don't like is that the bold doesn't hold open when empty. As a result I dry-fire the thing all the time after I run empty. Only way to avoid that is count your shots which I don't enjoy doing. If I had to buy again today, I'd buy a Mk2.