For a guidescope the most important factor is F ratio as that determines how many guide stars you see.
http://www.sbig.com/site/assets/file...ersion3_mb.pdf
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What this means is that, when it comes to finding a star on the chip, what’s important is the F-number of the telescope, not the aperture, and not the focal length. An F/4 system will see four times more stars than an F/8.
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