What are you doing with it? What GPM are you running? etc...... There are benefits of either one such as movable vs. not moveable, higher psi, fuel consumption variations .....begin with the end in mind and only do it once when your fuly researched on the benefits vs. negatives.
Gear drive is nice and compact, you likely will not break a gear drive on a 18hp setup. I am not a pump expert, but the little ones dont last too long on the lower hp machines, I doubt that they will be any better on a higher hp application. Belt is ok but more expensive and then there are issues of slippage, breaking belts and the like......its a personal preference thing. I like gear drive the best so far personally and have a 24 hp 8gpm gear drive and love it!