I've had my 6x17 Gaoersi for 3 or 4 years now, it's been very reliable and when I had a mishap with the viewfinder I had no problem getting the broken part. (The camera must have been accidentally dropped).
It was pure luck finding a good 75mm SA in Italy, it was listed on Ebay UK, when it arrived it was mint in it's original box and appeared to be unused.
I think for 6x12 I'd definitely recommend a 75mm over a 90mm. Have a look at Steve Smiths posts recently on his home built 6x12 with a 65mm SA.
Ian