I have an LDG RBA-1:1 current balun, and I was wondering if it behaves as a common-mode choke that I can use for a ground-mounted quarter wave vertical w/ 16 radials.
The other choice I have is a 4:1 unun (also LDG), but I think that would cut down my impedance quite a lot.
Unfortunately, I currently do not have an antenna analyzer so that I can properly measure the performance.
Looking at the pictures of the LDG RBA-1:1, it looks like the answer is yes. It is just a choke that can be used as either a balun or unun. Note that if the vertical has a resonant feedpoint resistance of 35 ohms, a 50 ohm unun will transform the 35 ohms to a slightly inductive impedance which will decrease the resonant frequency at the unun input terminal. Or if you want a perfect SWR, you can use a shunt capacitor at the unun input terminal.
You can. In fact currently I'm doing some experiments with verticals and do exactly this - use a 1:1 current balun which I originally made for a dipole as an RFI choke.
If you are interested in the subject, here are a few articles that will answer all your questions regarding baluns, RFI chokes and common mode current:
