I can speak for Saito & Magnum .91s since I own both.
First compare the weight to a .91 2-stroke, Magnum XLS or OS FX (I have and have weighed both of these)

.91 2-stroke 25.5 ounces

Saito .91 20.5 oz. (Digital scale weight)
Magnum .91 22.5 oz. (Digital scale weight)

I would think the OS would be the same as the Magnum, since the Mag is a copy. From what I read, the Thunder Tiger is over 26 ounces, more than the 2-strokes.

Power:

My Saito and Mag are about the same. I would put a friend's OS in the same area. It is said on RCU that the TT is more powerful than either of these.

Power-to-weight goes to the Saito. Ease of break-in and set-up goes to the OS. Price goes to the Magnum. I don't like heavy engines so I haven't considered the TT.

Some people are going to tell you they love the YS, and I'm sure they do. But, the regular fliers who I have seem with them have messed with them a lot. They run YS 20-20 fuel and they use a special after engine run oil. They do have lots of power and you can run the tank anywhere, but they are heavier and I am not willing to put up with the special requirements.

Check what guys are flying at your field and see how they run. You pays your money and takes your choice. The 2 brands I have and the OSs do run well. Any would be a good choice.

If you are used to ABC 2-strokes, you will need to do a good break-in on a ringed 4-stroke. It is not unusual for them to need 2 and up to 3 gallons before they really run great. I'm sure someone is going to tell you to put it in a plane and go fly, but check with the guys who fly 4-strokes a lot and who love them and they'll tell you they ran them some on the bench and then ran them rich in the air and it takes a couple of gallons for them to really run well.