Re: Which OSX version suits best for me? Tips?

Which version of FCP? If it is anything before FCPX I recommend you dual partition or have dual boot drive with SL and FCP7 on one partition and not upgrade that one. Just leave it legacy and working.

The other drive/partition should be kept up to date with Lion & soon ML to match Resolve & CS6 as they develop. Trying to keep new OS & software running with FCP Legacy will cause problems eventually.

Also if you are running cards like Decklink, Kona, Matrox etc, their drivers will need to be left for FCP7 but updated for other software & OS updates so another reason to run dual booting.