Differences between Outback inverters (GTFX vs FX)

So I'm killing some time tonight browsing the NAWS store and reading up on the Outback inverters. I noticed the FX and GTFX inverters are the same price for a given voltage/wattage. If I were to buy one of these, are there any major differences between the models that would influence that decision? (Aside from GT versus non-GT, of course!)

Thus far the only differences I've come across are that the GT series can't be stacked in as many different ways, and they have a tighter AC input frequency tolerance. Is there anything else?

I also noted the NAWS page for the GTs says "not recommended for use with AC generators". Why is that? Frequency tolerance? (In which case, would using an inverter generator with stable output frequency work okay?) The possibility of damage if Sell is enabled and you backfeed the generator? Or some other problem? I haven't found any mention of that in the manuals yet, but not gone through them cover-to-cover either. Just thinking what-ifs, if the grid is down and I need to recharge the batteries...

I don't currently plan to go grid-tie, but it's certainly not outside the realm of possibility. If I were to buy one of these, for the same price it might be nice to have the functionality available if it doesn't negatively affect another feature I may find more useful...