I was reffering to Airways Flight Training, as I stated.
In terms of weather the main advantage of exeter is during times of marginal weather.
Exeter being at only 100ft AMSL where as Bristol is quite often hidden in the clouds.
However if your planning on doing the course in the summer, if you get good weather most likely then haze is likely to be the issue, and it wont be much different at either.
Sorry that does not help much.
Simple answer I would say Late Autumn - > end of Spring Exeter is your better bet for VFR. (not guaranteed)
You pay your money you take the chances.