Several differences:
1. Documentation
2. Business Classes
3. Form Classes
4. Security

Even though CB61 is free you still need to do alot more work to get an application up and running. You can get the basic structure built on the same the time with either one - but getting the customization is a little more difficult in CB61.

Mere Mortals 6.1 does have a bit better documentation and explains how to use several of the form classes that come with it. Business classes have a much better GUI to it and plenty more options.

Security are the difference between night and day. The security in Mere Mortals is far more advance.

Which ever you choose it'll take to a whole new level of developing with VFP. Be prepared to do alot of studying...