I have both the 12" GSO and 16" GSO dobs so I think I can give some perspective on relative performance.
In relation to test figures,
I will give you this thread on Cloudy Nights to consider,
http://www.cloudynights.com/ubbthrea...9/Main/5559050
which includes a link out to a German translation of a full test of a 16" GSO mirror; and these 'old specs' from Andrews Comms.
The 12" Optics: 1/12 surface wave error (now claimed 1/16 RMS), BK7 mirrors (primary and secondary)
Resolving power; 0.38 arc second.
Limiting magnitude; 14.9
The 16" Optics: 1/12 wave surface error (now claimed 1/16 RMS, BK7 mirrors (primary and secondary),
Resolving power of 0.29 arc seconds.
Limiting magnitude of 15.5 .
If you would like to really dig into the whole world of mirror quality, then have a read through this article:
http://www.bbastrodesigns.com/ratemirrors.html
In recent times they have changed their stated minimum criteria to 1/16th wave RMS, or better, as RMS is gaining wider acceptance amongst the astronomy community."
Here is what GSO has to say about their minimum mirror criteria:
http://www.gs-telescope.com/content.asp?id=142
The 12" GSO maybe the best value in the production scope market right now (at $749). When you add some good eye pieces it might last you the rest of your life and never bore you.
If your just starting out, buy the 12 and you won't be disappointed. See how you go with that, it's a good scope, the only drawback is that the 12 needs a few mods (like a spring upgrade).