I am the happy owner of the 27/29. I have a cabinet running multiple emulators ranging from c64, mame, to snes, genesis, gamecube. My F-E is hyperspin.
The xgaming website explains it well: the 29 uses Analog adjustments, so you must manually change display settings on the monitor itself for any change in resolution/refresh. This makes it an unlikely option for a standard MAME user. The Tri-Mode monitor is ideally suited for replacement of an arcade machine which will be set to one of the Three Modes PERMANENTLY.
My $.02: if you want to run anything other than a select few games where you need the lower refr rates, go with the 27/29. I would go as far as saying, if you are posing this question, you probably won't have much use for the 29.
I'll admin that I am a bit biased, since I have the 27/29 and, in my view, it is worth every single penny. It gave me the right balance of simplicity/authenticity. You won't even need any special video card, pretty much plug and play once you figured out how to output your windows system/video card to 640x480 or 800x600.
Most (if not all) old games look fantastic, including things from the eighties. And certainly much better than any LCD monitor I tried.