is there point to use 6.0 as 7.0 is there?
It depends...
The 7U3 is the latest and supporting the most recent CPUs.
Hence, if your hardware is not more than 1-2 years old it would make sense.
Also, if you want to run clusters, then definitely use 7.0...
But feature-wise I don't know about any significant improvements made to the
hypervisor itself.
Considering the recommended (and reasonably priced) hardware for TrueNAS is 5+ years old, ESXi 6.x will do just fine.
And, as icing on the cake, if you stick with 6.0 you can use the old C# client (not web client) that is quite a bit more responsive...
And you can run 6.0 on a 1GB USB whereas 7.0 requires high-endurance storage like SSD...