If you only have a few blogs, then moving to mu is not worth it…in my opinion. I would much rather manage 20-30 individual blogs than one Mu install. Mu does lag behind WP in version upgrades and you will find fewer plugins that are mu compatible. From my experience, themes seem to pretty much work on both equally. Mu support (in the support forums) is pretty much non-existent compared to these forums, so if that is important, there is no comparison in responsiveness in the support forums…just go there and browse around a bit to see what I mean.
Don’t get me wrong…mu is an excellent piece of software. I’ve been using it for over 18 months on one site with over 400 users, but there is a big difference between managing a Mu site and a regular WP site.
My advice…stick with WP unless you are planning to host hundreds of blogs and want to allow users to create their own blogs on your site. Then, if you do move to Mu, don’t expect the kind of responsiveness in the mu forums that you get here.
Just my opinions based on my own experiences…take it for what’s it’s worth.