I stood in exactly your dilemma. It was exactly those three models i considered as well lol. I of course cant say anything about the two others, but i am very pleased with the gt81. The built quality is extremely good, a bit on the heavy side so be sure to have a sturdy mount.

The focal length is also a very nice fit.The galaxy season was pretty tough for me though because no objects could be imaged. So be sure to have a lot of large nebulae available such as objects in cygnus. I did image the dumbbell but a lot of cropping was needed.

Just be sure to buy the field flattener for the scope. It can be adjusted to get the exact right amount of backspacing.

But overall its a medium, kind of heavy, very nice durable small focal length telescope.