Can anyone tell me ??
There is so many that if I typed them all out I would pretty much recreate the Release Notes. Therefore I suggest you read the actual Release Notes for 9.3 Patch 6 and 9.4 Patch 2 and evaluate them for your own needs.
In a nutshell though: 9.3 = monolithic policy structure with DLP technology from a few years ago. 9.4/10/11 = multi-layered policy with rules in rule sets with current DLP technology.
I would not consider 9.3 now for any organisation. Use 10 or 11 instead.
I think it depends on what you want to achieve, they both do same things but differently, the schema in 9.4 is completely different to 9.3
The best option is to read both 9.3 and 9.4 product guides and Release Notes for clearer understanding (also 10.x for more broader understanding)
See links for more details to help
Hope this helps
If u can point out some major differences between 9.3 and completely different 9.4..