My small telescope is a Skywatcher 102 refractor which I bought as a cheap and light alternative to lugging around my 200mm reflector. It has a 102mm diameter and 500mm focal length equating to a f/5 lens. I also have a Sigma 150-500mm lens which is comparable, but higher quality. I wanted to see the difference between the cheap telescope and the expensive lens for astronomy so I took out the two for an evening with the moon.
The first advantage to the Sigma lens is that the lens has autofocus, while the telescope does not. The first immediate disadvantage of the lens is the slower speed and I cannot use any filters. All in all, I found the lens easier to use and got better results from it, however, the telescope is a lot more flexible in that I can add filters, use Barlow lens (for greater magnifications) and use it for eyepiece projection.
I think I'll stick to using telescopes for astronomy, but look at investing in a higher quality refractor.
Here are the two images for comparison.