Hey guys,

I'm facing a dilemma whether to replace auto transmission or not. My Mazda 3s, 2006 has 117K miles on it. The AT light went on a few days back and I took it to the dealership to check. They said that the error codes indicate internal problems with the 2nd and 4th gears (symptoms: when switching to those gears, the engine revs up to 5-6K RPM without any power, then the switch to the next gear happens). They are saying that I need to replace the transmission. I can put a new transmission (0 miles, ~3K USD for parts + labor, 12K miles warranty) or a used one (55K miles, ~2.1K USD for parts + labor, 3 months warranty). I'm looking for an opinion regarding:

(1) Which would you choose (new/used) and why?
(2) Does it make sense to get a second opinion here?
(3) Does it make sense to do it in a dealership or take it to a third-party shop? (concerns that I have here - it's hard to find a qualified mechanic to do the job and I don't want to end up paying more later. I guess that equally applies to dealerships as well as third-party shops - any insight here would be appreciated)

Thanks for your help!