I have the Little Gracie II. My understanding is that the metal-based frames tend to be sturdier and less likely to bow, or shake as you're quilting. Also, looking at the Mini Pinni, the rollers seem to be upgraded from what is on the LG -- my rollers are functional, but not necessarily top of the line. Looks to me like the rollers on the Mini Pinni are also thicker, so probably are less likely to bow under the tautness/tension of the quilt -- I've personally not had a problem with that yet on my LG, though. The Mini Pinni also comes automatically with their upgraded carriage, whereas the LG has the wooden carriage, which is not as smooth as the upgraded metal carriage (I upgraded recently to the metal carriage, and it's made a huge difference).
My sense is that the Mini Pinni is a smaller version of their top of the line product (the Pinnacle). The LG II is a good frame, but if I had it to do over again, I'd probably get the Mini Pinni...it's only a price difference of about $100, and you do get a couple of upgrades that are worth it (the metal carriage, metal frame, and the stronger rollers).
I'd definitely recommend checking out the Yahoo Group devoted to Grace Frames:
http://groups.yahoo.com/group/Gracemachineframe/
It doesn't get a ton of traffic, but enough people post there that you should get some answers.