My wife & a friend of mine have dove an old AGA Interspiro FFM. From what they told me, and what I've read, the following points are of interest:
1.) You can breathe through your nose.
2.) Equalizing isn't quite as easy, or at least requires some different technique.
3.) It doesn't fog of require clearing.
4.) They go through air a bit faster than using a regular mask.
5.) It's secured to the head well enough with those straps that just 'whipping it off' wouldn't be real easy. This is an issue because...
6.) The AGA doesn't have the capability to pull off the 2nd stage and accept someone else's octo. in an OOA emergency. And pulling the whole thing off underwater can largely blind you, so you'd better stay on top of your gas supply diligently (which you should do anyway, of course).
7.) Some FFM's let you add a com. unit for conservation (for a nominal fee, of course).
You're looking at a different product, yes, but I wanted you to know my frame of reference in suggesting these points, if I were looking for a FFM for myself today, I'd want...
1.) One that would let me breathe off someone's octo. without pulling the mask off my head.
2.) Maybe one that had the option to add a com. unit in case I ever did want to converse with people underwater. Right now I don't, but there are 'challenging' dives where it could be a plus.
While $600 is not chump change, a FFM is a big enough investment, and important enough, I don't recommend making much compromise on a tight budget. If you need to wait till you've got more money, wait till you've got more money. I'm not pushing any particular brand. I'm just saying an $800 mask that an excellent match for you is far better than a $600 mask that's tolerable with flaws.
Whatever you get, post back sometime and let us know how you like it!
Richard.