The only real advantage with a mid-mount is they are physically shorter, and can get more of them on a 16' landscaper's trailer, hence their popularity with LCOs. Mid-Zs have a much wider range of classes with everything from entry-level (aka semi-disposable) consumer to top-of-the-line, beat-em-up-all-day-every-day, heavy duty commercial. I think they also run a sizeable bit less (with the exception of maybe the diesel, propane, and LNG powered ones,then it's probably negligable) than a comparable quality front-mount ZTR, but usually the front Zs have greater flexibility of uses and attachments. The industry has stepped up and made many attachments for mid Zs (snowplows, dethatchers, spreaders,...) but I think the front-Z still gets the nod. Front-Zs offer the ability to reach under low tree branches (pines) much more effectively and easily; and their cut may arguably be a little better since you're not driving a heavy caster over the grass before cutting it.
All that said, I ended up with a mid-Z (DC LT2700-60D) when I was last shopping. Granted it's not really a directly comparable machine to any front-Z, but it more than met my needs, and was at a price no front-Z could remotely touch. After 6 years, I have no regrets despite still thinking front-Zs are cool.