I have used the grey felt ones on almost all of the 30 machines i have, and I have not had any problems with any of the redecalled machines either.
I note, when I have moved machines from premises to premises, and have had to take off the protectors (cause i took the legs off) , there were not any sticking or damage issues from the felt protectors.
I also note, that the plastic style protectors that I recently saw on the RTBB site for example, do look alot better, but have seen some slight indentations from the hard edges on a friends machine. But, I have to say, if u never remove the protectors, you will never have an issue. I mean, why would you not keep them on. They do come in different colours but are more expensive.
My advice, if u want to save money, go the grey felt ones.(soft side goes towards the machine).
If u want to add the bling, and dont mind spending the $, go the RTBB ones.
hope it helps.