Well, a 250 2-stroke will have about twice the power of a 125 2-stroke.....
If you are comparing a 250 4-stroke to a 125 2-stroke I would go with the 250.
As for having control, going to 250 doesn't change the control you have. Not having the power to mis-use might make your lack of control easier to deal with on a 125. On the other hand, having power to spare with the 250 might get you out of trouble. Opening the throttle a little wider is easier than shifting down when climbing a hill.
You say you have ridden an 85 and a 250.... so a 125 would be about in the middle! How much riding did you do on the 250?
I have riden a number of different Yamahas, 2 and 4 stroke, "TT" and "YZ". I am not that good of a rider and I certainly don't race but I do tend to push my abilities, some times a bit too far. I find that the YZ suspension is a lot more forgiving of my mistakes. Let's just say that I managed to make it over some obstacles that I really didn't want to go over in the first place.....
My bike is a YZ-125. It and I have an understanding, I treat it well and it has been treating me well. It has only thrown me off once and that was all my fault. My son has a YZ-250. My mistake when I rode that bike was that I treated it like my 125. It definately has the power to get me in over my head much more quickly than my 125 does. I think that once I train my right hand to deal with throttle settings between WOT and Idle I would do a lot better on that bike.
I have ridden a Suzuki exactly once, I think it is a 225 2-stroke. It was a friend's bike, and I can't keep up with him when he is riding it. But that bike and I just don't get along. I don't know what it is but I would be riding along thinking things were just fine and suddenly WHAM! Thank god it was a soft sandy trail! I don't know if it was the rake of the forks, the wheel base, where I positioned my weight, or what, but that bike dumped me three times on a trail I had ridden a bunch of times before.
Rod