I've never had either, went straight to the 70-200 f/2.8 II. But looking at TDP's IQ comparisons, you might see an improved IQ, but it seems the "IS" version is only a very little bit better. You may not see the improvement in IQ.

So, it would get down to 3-4 stops of IS. I do find IS very helpful for tele lenses. Shooting 200 mm at 1/10-1/20, but more commonly, even getting to hand hold shutter speeds of 1/60-1/80 is very helpful. Then, you should be able to shoot 70 mm at 1/5-1/10 and still get sharp images, but having near perfect keeper rate at 1/60.

If you are missing shots because of needing to bump ISO to get shutter speed, I'd look at the move. If not, maybe the money may be better invested elsewhere.