Yes you can buy either. They are physically the same size. The difference with polyurethane is it doesn't degrade like standard (cheap) rubber bushes. They tend to be a harder shore (Higher numbers indicate harder materials; lower numbers indicate softer materials). So you will feel more vibration transmitted through the chassis to you.
I bought standard rubber ones (generally from past experience they tend to be about 40 shore) as I plan on daily driving through lanes, lots of off/on throttle, overtakes, etc. So I'd like to retain a bit of "softness" to my power delivery. Polyurethane ones tend to be 60-80 (I'm dredging this up from ten years ago so may have changed slightly in that time).
Source:
https://en.wikipedia.org/wiki/Shore_durometerIf you want to get real fancy you can get hydra bushes. I very much doubt they exist for old Polo's though. They also like to burst and feel shit when they do.
