Although I would choose Subversion over CVS in most cases, you should know what you're missing with Subversion:
CVS sees tags and branches as different things; Subversion doesn't. This means that third-party tools built on top of Subversion (e.g. IDEs with source control integration) have a harder job knowing the difference. You usually have to do some special configuration to tell it where your tags and branches are, and you have to make sure that your users stick to a certain filesystem layout.
Subversion can't look at a file and tell you at what point someone created a branch or tag from it. Tools like CVSGraph can use this information to draw a tree of a file's history. To do that with Subversion, you'd need to search all the branch/tags directories, and I haven't seen any tools that do this well.
CVS has been around longer, and the third-party tools are more stable, in my experience.
