Debbie, I'd jump up and down with joy if I had to make your decision. Either of those is truly a sweet scope. I've had a little more experience with StarMasters than Obsessions (not enough with either), and I really really like them. You could make the argument that a thinner mirror reaches thermal equilibrium more quickly and a thick mirror is more stable, but we're talking cream-of-the-crop in either case. If you can't lift the Obsession, then buy the Starmaster. You'll never have a day of regret.

What are the advantages of each and which ones to your guys/gals prefer? Is there a visual advantage to a thicker mirror or are they about the same?

I was thinking of upgrading to an 18" dobsonian reflector. The StarMaster 18" telescope seems a little more portable than the 18" Obsession. I can't lift the Obsession 80 lb mirror box. I'd need a trailer or truck to haul the 18" Obsession.
Which would you guys/gals pick and why?

Debbie