The rotary cutter will be better if you let the grass get tall and only mow once a month or every other month. If you will be mowing on a regular basis, then the finish mower will give you a nicer finish and is easier to operate. The gauge wheels allow it to follow the contour of the ground better then just a tail wheel on a rotary cutter. Another advantage to the finish mower is that you can go a little bigger on it because it takes less power to run.
The basic rule of thumb for a rotary cutter is five
HP for every foot of mower. With 18
HP, 3 foot is ideal and 4 ft is pushing things.
If you will only have one mower, then I think the rotary cutter is a better choice. Especially if you are like me and let your grass get out of control when it first gets growing in the spring time. It's just too wet to keep it under control, so I end up with a real mess at first. Then after I get that under control, I switch to my finish mower and keep that on pretty much the rest of the year.
Eddie