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There are other factors for guiding. I used an ED80 and then an ST80. I found the later was superior for many reasons, although it was not suitable for imaging. The ST80 is widerfield, never short of a guide star, and its is also much lighter than an ED80. This means more stable guiding on a marginal mount (less loading) and less differential flexure, which for me was the critical difference. With the guide cam bolted hard to the ST80 and scope with no rings and no ability to refocus (not essential, once set) I can get 15-20 min subs without obvious flexure (I couldnt get this with the ED80).
With CCD soft and Maxum DL, I found guiding accuracy is fine, even tho the ST80 has a shorter FL than the ED80, with a long FL imaging OTA (>2m).
Of course the ED80 is 1st choice if you wish to image with it too, but purely for long exposure guiding performance with an average mount, the ST80 is better (and cheaper).
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