I have the unlimted $19.99/month version. I do alot of internet browsing and I do not want to worry about usage limits. Also I do not currently have a home computer. :-( So for me $19.99 is a reasonable charge.
I also have the MediaNet Unlimited for 19.99/mo but use the cu500 (no "v" version available a few months ago). The cu500 has been fast enough for me but I don't use it as my primary internet connection. (You can search this forum for measured speeds.) I'd recommend going with the fastest phone available. Right now, it sounds like v3xx is the phone you want. Any Sync users out there?