Hi there,
Recently I had to format my computer, what means I have to reinstall Quest once again. As I'm not in a hurry, should I install version 5.8 or should I wait for the release of Quest 6?

I think 5.8, Q6 may be a while before it's ready.
Then again, go with both and be prepared.

If you're working on existing projects, 5.8

Quest 6 is a completely different system