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The GSO and Synta Dobs are a great bargain for the aperture. If you can afford a 12" Dob, go for it.
There is no reliable objective evidence to support the claim that the 8" mirrors are higher quality than the 12" mirrors from these mass produced vendors. A shorter f ratio mirror is a bit more work to make well than an f6, so yes the median 12 f5 could arguably be less well figured than the median 8 f6. Nevertheless, you will find hundreds of GSO and Synta users here and on Cloudy Nights who will testify to the great enjoyment quality from these scopes. I have an Obsession and a TV refractor, and the 12" mirror may have been a tad less quality than those units, but to be honest, you have to work to notice that. Furthermore, the quality of these mirrors has improved mightily now that there is competition in the market AND because the internet (that's us, with our giant implacable information network) keeps them honest. Put out consistently poor product - not just an isolated flaw, everyone has bad days - and word gets out quick.
The 12" f5 will be a bit more touchy about collimation than an 8" f6. You will need some collimation gear either way, but for a given degree of mis-collimation the 12" will be affected more than the 8". That said, it is an easy thing to learn.
The biggest issue is the mass. A 12" is considerably more work to haul and set up, even tho 4" sounds like a small difference. The best scope is the one you use the most.
Scott
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