I've been using Sitecore and Umbraco for a few years now, as well as occasionally going back to RedDot / OT. I've got to say, they're VERY different kettles of fish. RedDot's really good at publishing out flat files, whereas CMSs like Sitecore are much more dynamic. The fact that you can query the actual content DB means you can produce things like APIs to query lists and so on. Listing pages can be generated dynamically using javascript and AJAX, for example.
I had a bit of a stepping stone via a CMS called Immediacy (which has long since been named other things - I'm not even sure what it's called these days), so that helped me get into Umbraco and then Sitecore. One thing's for sure, though, if you're still using good old ASP and VB script in RedDot, you're going to need to look into C# instead.
Still - I'd say go for it. Nothing wrong with selling yourself as a Sitecore / Drupal dev AS WELL as an experienced RedDot developer :)
Good luck!
Mark