I have lots of chucks, both key types and keyless types. Most of them are high quality and in excellent condition. I find myself choosing between a key or keyless type by the kind of work I will be doing. For heavy duty, rough work, I nearly always choose a key type, usually a Jacobs ball bearing super chuck. For lighter work, I usually choose a keyless chuck, usually an Albrecht, Rohm, or a Spanish made SPI. Keyless chucks do work just fine for heavier work, but are sometimes more difficult to loosen the drill after the heavy cut. Ball bearing key chucks loosen easily, but there is also a key involved. For average one off work, I just grab the first chuck I find that will hold the drill and fit the machine. Often, the chuck is chosen strictly for what kind of shank it has. For the mill, I usually prefer straight shanks, for the reason given by Eddyde above. I do have R8 shank chucks, but usually only reach for them if all the operations to be done on the part can be easily done in one setup. Straight shanks are a lot faster on the mill, especially when there are a lot of different tasks that must be done, and even more so if the space is tight. I try to avoid moving the knee or quill up and down large amounts for the various jobs, and for changing out the tooling. With a little thought, and planning ahead, it all goes a lot faster and with a lot less effort.
Be careful about buying used chucks (and everything else) on eBay. A lot of sellers dump their junk there, where buyers cannot test before purchasing. If it turns out to be a bad tool, it can be difficult (and a PITA) to be made whole again, not to mention the wasted time spent. It is useful to ask highly specific questions to the seller. No answer, or no answer that gives you confidence you can hold him to it, then no purchase. Life is too short to be spent chasing liars, scams, and scammers.
