All of the above. Fit, feel, looks, the way it carries, etc. I started out with a Winchester and their straight grips but have came to like the Marlin for its better design IMHO, and those that started with the pistol gripped Marlins will probably favor that style.
It's a definite comfort issue with me, and I prefer the pistol-type grip, but have guns with both pistol and straight.
I just find the more acute angle on the pistol grip to be more comfortable because I don't have to keep my wrist bent at a funny angle and can maintain my grip for a longer period in a shooting position.
My wrist just doesn't bend to an angle that likes a straight grip for long periods of time.
what fw said also there is the school of thought that you have more straight back push & less muzzle flip with a straight stock & get back on target quicker, i havent shot enuf straight stocks to tell a diffrence only own pistol grip,mike243