I borrowed a Keysight B2980A for evaluation. It was much nicer to use than a Keithley 6517B. Unfortunately, the key thing with an electrometer is its input leakage current/offset, which can be evaluated by capping the input and logging for a while. The B2980A didn't do well, so I bought a 6517B despite its appalling user interface. I later also bought a 6430. See the graph for a comparison of their drift.
The 6517B is sensitive to mains transients (you can see some disturbances on the graph). The 6430 is much more tolerant, possibly because of its median filter, or perhaps its power supply was designed better. The B2980A has a battery power option, so unplugged from the mains and on battery power, it should be immune to mains spikes. Realistically, a decent electrometer ought to be immune to mains disturbances, so in that respect the 6517B is flawed.
The good thing about the 6517 series is that they have a "2V OUT" connector that you can (and should) monitor on an oscilloscope to prove to yourself that you are measuring a genuine current, not the DC component of distorted mains hum. The 6430 does not have that option, so you have to take its numbers on trust.
Finally, there are four versions of Keithley 6517; 6517, 6517A, 6517B with vacuum fluorescent display, and 6517B with LCD display. The 6517, 6517A, and VFD 6517B all suffer hum (don't know about the later 6517B), with the 6517 being the worst because they put the EI mains transformer close to the input electrometer!