I prefer rubber either red or grey.  The clear variety are unreliable - they seem a good idea for stamping sentiments in just the right place but I've ruined so many almost finished cards with them that I don't risi it anymore.  Even the good quality clear stamps seem to go funny in the end - some of mine have gone hard and some are like jelly.  At least with real rubber you know it will last forever and withstand heat etc.
I had a really bad experience with a new set (a £14.99 one) of clear stamps recently.  Spent a whole afternoon trying and trying to get a good impression and really couldn't.  The stamp not only didn't print the whole image but also printed a hair line.  When I looked closely there was line on the stamp so I rang the company and they replaced, but if it hadn't been for the line I would have just have thought it was my lack of skill and not a faulty set.  It would take a lot for me to risk buying clear stamps now as I just don't trust them. 
Ann xxx