If you want to study, 7.0 is the best choice. Many functions of the new version have changed. Many codes in the previous books will go wrong, so you may not be able to run other people's toolkits.
If you already know Matlab well, use the latest 20 12a or 20 12b, which is more powerful, supports parallel computing, and is very powerful for handling a large number of calculations. However, I don't think there will be a problem that requires you to exercise for several hours in the competition.
Matlab7.0 has been relatively stable, of course, there will be mistakes occasionally. I have encountered two or three times (for unknown reasons), but as long as your character is not too bad, there will generally be no big problems.