What are criteria to compare and evaluate different 12 inch truss Dobsonian telescopes? What are criteria to compare and evaluate different 12 inch truss Dobsonian telescopes? I'm primarily interested in comparing mirror quality and weight (price I can do myself of course). Two examples of the category I'm asking about are the Sky-watcher 300P Flextube and Meade Lightbridge.

One of them appears to have a fan to cool the mirror, how can I evaluate that in terms of a positive or negative towards performance?
The cooldown fan is purely to speed getting the primary mirror to the local ambient temperature. It is not used - and should not be left on during observation, as it'll undoubtedly be a vibration source. 

That shouldn't be a significant factor in your final decision.  I would go with total weight, and perhaps adaptability to an equatorial mount for star-tracking.