I would demo them both to see what works best for you. I personally like a mid mount deck ZTR better, but if you have a lot of low trees to mow around, an out front mower may be better for you. There are a lot of posts on specific brand names and also a ZTR search thread on my Search Page which is the middle link under my signiture. There is hours of reading on this search page. Just click on the subject you want to read about and it will take you to hundreds of threads and thousands of posts. I hope this helps.
I run both, and while I really like the three wheeler (Gravely 300) for doing trim work and reaching in under bushes or those real low cut trees, it just doesn't cut even CLOSE to the amount of grass as a Lazer.
3 wheelers are very popular in my area but ztr's are catching on slowly. I find the 3 wheelers to be too long, slow and they don't turn as sharp. On the other hand some of the ones around here seem to last forever. The design is simple and solid. There are guys here using ransomes that are 15+ yrs old and cut everyday. I heard of a guy that had 8000 hrs on his scag 3 wheeler. Don't know how many times he replaced the engine.
Shane I have been using a three wheel Gravely with a 60" deck for about 6 years. It works great for the accounts I currently have, older established neighbor hoods with mature trees and shrubs. It works great for getting under trees with low branches. If you are doing wide open areas I would go with a Z mid mount. I plan on adding several Z mid mounts this year for accounts I've targeted that are larger and wide open. The 3 wheel mower will be used primarily for trim work around beds trees and buildings.
I have a friend who uses the scag three wheeler, but this yr he purchased a Z-master and i see him on this machine more now. He has over 4000hrs on that scag. If my job has alot of trees to undercut i first start with my 52"walkbehind, get all those done. Then i trim along the fences,around trees ,sidewalks,driveways ect, with my Trimmer.Then hop on my Z-Master and finish all cutting ,Grab my backpack and blow, But i don't have many accounts with alot of trees to undercut. I think Joes scag only runs 6mph and i like to cut much faster. But like Eric said it all depends on your yards. Marks Mowing Service
I have a 3 wheel rider. (Ferris) I chose it for many of the reasons mentioned. Good under trees, good for triming, Simple design, Easy blade change. built like a tank. The only drawback I can see is they take a lot of room on a trailer.
BTW it is not Zero turn but it will do a 180 in 4 in. I routinely mow around a 4 to 6 in tree in one pass.