In the range of vacuum that we are dealing with, exposed elastomer area, availability and ease of use will be the key drivers.
For small fittings (16-40mm) KF fittings are easy to use, plentiful, reliable and fairly inexpensive. In sizes over 50mm, the choices are ConFlat (actually ConFlat is a Varian trademark; the generic version is called CF) or o-ring sealed ISO. Both are a bit of a pain - I hate the ISO clamps but the tightening regimen of all those screws in the CF is also not something one wants to do all the time. With regard to cost, all of these standard flanges are made in such huge quantities that there really isn't a whole lot of difference.
Something that I do regularly is use viton gaskets in many of my CF flanges. You can either cut the gaskets from viton sheet or buy them from folks like Kurt Lesker. This is especially good for initial experimentation. In fact, with the viton replacements, you don't even have to use all of the screws. Every other or even every 4th usually works.
The viton CF gaskets can be reused many times and when/if you decide that it's important to minimize elastomers, then the copper gaskets can go in.
Steve
Created on Wednesday, March 07, 2001 7:39 PM EDT by Steve Hansen