I’ve been with textpattern blogging system; however, I felt that it was time to move on.
The actual motivation to move was that:
1. movabletype is well supported by movabletype developers. It’s funded product, so product has to be much better in shape imo.
2. There seems to be more users. I don’t know and count the exact number of users, but I’ve seen so many damn blogs made with movabletype. More users = More support
3. Six Apart makes regular updates. In the mean time open source project like wp, txt, etc does not have definite schedule when the next patch or update will be made.
Sometimes paid products are better to choose if a company does their job well. Funding means there is a support and there are developers working on the product all the time.