In your examples they are the same thing. Often only and just are the same thing.
However, the difference is that just is informal and should only be used in conversation. I teach my students to never write just.
The new television is just for DVDs and not VHS.
This is OK in conversation, but unacceptable in formal writing.
In conversation just is very popular, and often much better than only. For example, it would sound strange if Lady Gaga said "Only Dance."
Hope that helps