>> Stock WA M9s will work with propane for a longer time than a stock SVI, but not indefinitely.
That's counter to every piece of evidence I've ever seen or read.
Anyway, someone mentioned the flow restrictor. AI's doesn't fit the M92, but you could make your own (it's just a small disk that fits in the back of the blowback unit, with a small hole in it - It will reduce the gas directed to move the slide back, hence reducing the impact and stress on the slide).
If you're just plinking indoors and want a great looking gun, get the WA. If you want a cheap gun to drag through the mud on skirmishes and need the fps from Propane year round (WA's run fine in cool weather on Propane/Green as do any other gun, it's all about pressure which is affected by ambient temperature - Propane/Green have no magical properties!
), get the KJW.
I'd also chip in a vote for the HFC, though, it's a better gun, by far, than the KJW.
Cheers.