because they are two things a little different in the sense that v6 is structurally more sophisticated than v5 and "acts" in the cloud while v5 can also install it on a single workstation (stand alone). of course the v6 is more expensive and allows wider things than the v5. I've done nothing to get too much space.
news of the last hour: catia v5 will be active until 2024.
therefore, as anticipated by the prof. of the noble is more convenient and manageable than v6.
I saw turn v6: nice, maybe more flexible and above all solved chronic problems of v5 (correction measures without obligation to erase the unit of measurement, etc...), but you do not know where your projects end (except creating a personal server not in windows environment).
mastering catia by examples (mce) is an interactive video+tutorials course that will allow you to quickly learn the basic features to be really operational in a short time, at least for me it was so