I understood there's a new focus system on the new camera. But I understood as well you're doing focus bracketing and manual focusing to get comparable charts which is fine to a certain point. The thing is, if a lens is really good but the AF of one body less reliable than the one of the other one will not see the quality of the lens? But what you test is not AF accuracy or speed, so there's not much difference between an old, slower body or a new, snappier one?
Another aspect for changing bodies: nobody will buy a 50D these days if it comes to buying a new camera. I don't see a big reason in slightly higher MP but I do see a reason in different focussing technologies. Maybe I'm wrong.
A lot of lenses coming out are getting excellent resolution with the 50D. Who's to say that the 70D might not push some of them down a little, separating the men from the boys so to speak.
One simply can't test all lenses on all available cameras - ever available.
You can countercheck on DxO. They have a wider range of bodies to choose from. It's not an in-depth testing with sample pictures like here, but contrary to PZ they are asking money for their software so they do have a different financial background.
So, if you see a lens here tested with 50D, you can look if DxO did also test it on 50D and on 70D. The difference between the points gives you an idea if the lens is improving with another body.
And make yourself clear: A lens will never get worse on a better body. Only the pictures/results could get less sharp. So, one should always remember, the PZ test results are valid for the lens AND body combination. Some aspects would remain the same on a better body - like vignetting or distortion. Others could improve. And things like higher dynamic range which is improving the picture's contrast, has nothing to do with the lens in front of sensor.
I would vote staying with 50D, unless you are ready to retest all lenses.
15-20MP no big difference, minor differences in sharpness that will be shown between the two cameras will never be seen in real life.
It seems as if Canon will announce the 7D II at the photokina. That should be a good landmark for a change I guess.