To be honest it is not a huge upgrade from what you have now. You're already familiar with the SZ-1, so it's double that. Pretty cool sequencer, as you know. It's pretty much double a CZ-101 as well. In Tone Mix you get two voice polyphony, woohoo. You have better control over vibrato in a patch. You can disable it completely with the switch by the Mod Wheel, and you have a Mod Wheel

You get 32 user patch memories, but you cannot use the cartridge as extra memory like on the CZ-101. You always have to load what is on the cartridge into the user memories, which replaces all 32 of your patches

Oh and it has chorus. Which is really not a great one. It's just OK. Almost anything else is better.
It is exactly the same DAC and op amps inside as the 101, so there's no difference in sound quality as long as it is calibrated to factory spec. There's a simple DAC offset adjustment in the service manual. IME it is the number one cause of shitty sounding CZs, and it is an easy fix as long as you're OK with disassembly- you have to lift out the keybed and one large board to get at the trimmer for this IIRC. So maybe check the electrolytic caps while you're in there?
CZ-5000 uses batteries to retain memory in the same way as the CZ-101, but they're AA so much lighter! Won't make much difference, this is a heavy synth. Not as heavy as the FZ-1, but it is solid.
Obviously you get up to 16 note poly compared to the 101's max of 8. And honestly, there are far more sounds available using two lines than one, so most of the time our CZ-101s are 4 note poly. But like the 101 the CZ-5000 is not polyphonic via MIDI when multitimbral. You only get 8 monophonic channels, twice the 101's 4. But it is properly multitimbral with its own sequencer. Ya, these are the strange things manufacturers did in the early days of MIDI! Oh, and all the sounds come out of one output. On the CZ-1 you have two assignable mono outputs, woohoo! The CZ-1 is the beeskness of CZs. If you're a fan of the sound you will love that. Better than a VZ-1 in many ways, although it doesn't have the dynamic range of that synth.
What I'd love to know is if you can load a sequence saved to data-tape on the SZ-1 into the CZ-5000 sequencer, and the other way around too?