These synths are beyond compare as they are completely unrelated in architecture and usuage. The only similarity is the availability of sliders etc for realtime control.
The JD-800 is the machine that launched the JV series essentially and is based on JV type sample based waveforms. It will also be much older (naturally) so be careful to check all the controllers are working OK. Being a sample based machine, it can create a range of real instrument sounds (piano etc)
The JP-8000 is a virtual analog synth and due to the processing power involved offer fewer voices than the JD800 and aren't based on samples, but are computed realtime. Good arpeggiator and midi sync control. But don't expect real instrument sounds of it, it is indeed a virtual analog synth, and just makes analog type sounds.
If this will be your only synth, then the JD-800 will be much more flexible in terms of numberof voices & sound range.
If you are just adding it to an existing arsenal of synths, then go with the JD-800. It's newer and less likely to have hidden problems. Plus it has a much more advanced midi implementation.