The type of mount can influence your decision too.

Example: Seven years ago I ground a 10 inch f/6.3 mirror and used it in a dob. Worked great! Wonderful visuals of nebulae, galaxies and even planets when viewed with a dob driver. I remember watching Io follow the red spot all the way across Jupiter with images that rivaled the best ccd images!

Two years ago I went heavily into ccd and thought that if I could get a 6 foot long x 12 inch dia. aluminum tube....

Well I built it and put it on a GT1-HD mount. It's a really beefy mount but I soon discovered that it shakes in the wind. Too much sail area. Also, at 1600 mm the fov is too small, even with a Starlight MX7C. On top of that, it's a BIG scope!

So now I'm grinding a new mirror 10 inch ~f/4.5 and planning on cutting that tube down by 18 inches at least.

In the end, if tube length ins't an issue, make it as long as you can handle, pack and use comfortably. The smaller secondary will yield better contrast. On the other hand, if the intent is to use it on an equatorial, for ccd especially, then look at the very short f/ratios to both improve stability and increase the fov.

Tom Picciani