I use both blogcatalog and mybloglog. I started with mybloglog first and it wasn’t until a few days ago i signed up for blogcatalog. Already the traffic i receive from blogcatalog is greater then mybloglog. Blogcatalog has a faster user interface and a more involved community. They also have a blog rating function, where other bloggers rate your blog on a scale from 1-10.(im at 9.2 or something)
Mybloglog limits the amounts of daily interactions. You can only message 15 bloggers a day. Same limit with community joining and contact adding.
Blogcatalog has no limit and you can add and message as many as you like. This is a great way for beginner bloggers to get some traffic. The best thing, its all free.