Saleratus

You need to include the distance from the little mirror (called the "secondary") to the eyepiece. Your aperture is the diameter of the big mirror (called the "primary").

You don't necessarily need to bin your H20mm, but you could do better if you have the funds.

One more thing, if you are a serious perfectionist, you can look into the Catseye collimation tube. It is precisely adjustable according to the f-ratio of your scope.

There seems to be a bit of confusion here. Or maybe I'm the one who is confused. Anyway, this is my take--

When you have a long tube on a Cheshire, it's a combination item. It's combining a sight tube and a Cheshire collimator in the same instrument. I would rather have these separate, because the sight tube is used once in a blue moon to align your secondary mirror, while the Cheshire eyepiece is used to collimate your scope maybe every time you take it out. There is no reason to burden yourself with a huge tube when this is something you will be using so often.

So I'd get a small, nicely made Cheshire without the tube, and if you are a real perfectionist and feel the need to align your secondary (which probably doesn't need aligning), you can get a sight tube as well.

For me the decision would come down to what sort of nature do you have with regard to astronomy. If I could only have one scope, I'd have something around 8". I currently have 8" and 11" scopes and I use the 8" much more often, just because it's easier. I'm not 17 any more, but I can move the 11" when I want to. It's just that the 8" is so much easier that I know I can set it up and take it down without any fuss. An often repeated saying in amateur astronomy is: the best scope is the one that gets used the most.

However, if you have buckets of enthusiasm and energy and fairly reliable weather (so that you don't have to worry about rain rolling in and making you take down your scope 10 minutes after you set it up), maybe the 10" is for you.