Vps hosting provides choice of operating system:
Which pros and cons for running rails app, software installing, overall feel, etc. do they have, or maybe there are others which are much better to serve rails
These are all industrial strength unix OSes and will all run RoR on with Apache extremely well.
The main difference is ease of admin and ease of mainteneance. Cent OS is designed for remote server installations and has lots of nice management features, Ubuntu is probably the eaiest to maintain -- it just sort of happens.
I would go for Ubuntu 8.0 over Ubuntu 9.0 as the older OS is has proven to be extremely stable. (In fact for server apps stick with the Ubuntu "long term support" releases unless you like applying patches!).
Ubuntu LTS releases have a good track record, and many other rails users use ubuntu so most blog posts etc will be easy to follow.
But since you are going to be digging around in there often, overall I would recommend you use whichever you have the most experience with.