We have fitted a lot of C and E series sets over the last year and have been very impressed by them.
Observation would be that in most cases the C series is more than enough for a RHIB yet a fair bit cheaper than E series.
E series has advantage (nothwithstanding 3 D mapping - not essential) of being a full blown computer and from that has significant applications that may suit specialist vessels. 1. Ability to plug in video cameras . We have experimented with underwater units and this has potential.. 2. Easy to link several unit together ie. a number of pods on a vessel sharring info for training purposes. 3. Ability to show engine diagnostics on the screen. There are no doubt other abilities that we have overlooked.
We are now playing with a SAt transmission system whereby you can "ping " the boat and poll back all engine data via internet/satalite/e-series/engine EMM. Requires another couple of £k of equipment from another company but works well worldwide.
However unless you are into/have a need for all these funtions then C series is the recomended. In your case you have afforded the E series so enjoy !