I have two engines in which I need to pick to put into my Tower hobbies 60 trainer. I have a new K&B 61 two stroke and also have a OS 91 FS. Which engine would be the better choice. I'm wanting a good idling and smooth runner.

Both great choices. How well they run is more about how well you can tune and engine. Another thing is the OS may work out better if you require a bit of extra weight in the nose.
The trainer I have on hand at the moment has an OS .91 four stroke in it. My other engine choice was a SK .80, both good engines but I enjoy a four stroke more then a two stroke. It's a choice thing. Both your choices are good and will work just fine.

I have flown my TT 60 on a whole bunch of engines. For me, the best set was an older O.S. 70fs. It sounded great in the air, plenty of power and would cruise around all day at 1/16th throttle. Just floating around at a high idle.

Either will pull the plane around very well. I flew mine with a K&B 65 for many years. It took off in about 3 feet and was way over powered. But still fun.


Buzz.

Put the 60 in it.. put the FS in something more deserving of it

I have a Saito 91 in my 60-size Fun-Fly plane and LOVE IT!!! Easy to set up and I almost never have to touch the needles, just fill and fly.

However, this is not my "beater" setup. So, if you are going to use it for learning, go with the K&B61. It may be a little more difficult to set up and run (my K&B 40 is pretty sensitive), but you wont feel as bad if it gets filled up with dirt