My preference: Cona. Stove top is the easiest.
I don't like the electric vac pots - they are very finicky and I never managed to get mine to work properly. Sold it to Hugh in the end.
I like to use vac pots on my gas hob using a heat diffuser. I always had fairly good results from the Santos, and can recommend it in general. However the Conas give even better results. The step from the Santos to a Cona is almost like the step from a press pot to the Santos: quite startling changes in brew flavour and clarity.
So I recommend the Cona. A "kitchen model" is more than adequate, and in fact more practical, unless you want to impress your guests at a dinner party. You'll pay a lot more for the "table" version though.
As you say there are often Conas to be had on eBay, although often as not they're the table models and can go for quite a bit.
Brand new Conas and Santoses are both available from Hasbean. That's where mine came from. I watched eBay for a suitable pot for a while and then gave up and bought a brand new one from Steve. They're a little bit more than the Santos, but not much IIRC. They're more "solid" too. My Santos broke eventually during normal use. It developed a crack in the top bowl (I don't know how) and eventually a circular panel of glass fell out. I can't say for sure but my feeling is that the Conas, being made from thicker, heavier glass, will last longer.
Neither will bounce though!
HTH
Phil