I have also just recently started playing SH4 v1.5 with RFB 2.0 and RSRDC. Played SH3 with Grey Wolves a fair amount over the last couple of years and really enjoyed the experience...both with 100% realism and DID.
It's a little too early to say which I like the best, but the SH4 experience really is great! I wanted to try the PTO with a US boat...go through the learning curve as a rookie skipper starting from scratch again...also wanted to experience some of the nuiances of sub warfare vs. Japan and with another mega mod.
A couple of years down the road I'll probably think about SH5... after the bugs are squashed and the talented mod community is allowed to work their magic!

.
I've by passed (so far) the experience of using the PK to update target solutions...constant bearing approach is my tactic of choice early on in my PTO career.