I use the RC42 Heading sensor. If I recall, it does more than what you are discussing. One if the things it does is know which direction you are pointing and travelling. I forget all the stuff it really does, but it was for sure worth the investment.
I have used Marpa a few times. It is pretty cool when heading in from 50 miles out, others travelling in as well, I can select those boats and track them to determine if I might have a potential collision issue, then I can make adjustments.
It will be better when/if Navico moves us up to ARPA so we don't have to do it manually. I had heard rumor they were working on it as a SW upgrade down the road.
That 3" looks nice and seems it would work for you well enough!
The Velocity track is awesome. I can watch cars go by in front of the boat when sitting at the coast and see the colors change as they travel by. There is a large open field behind me and I was watching the riding mower go around and the Velocity track was keeping track and showing when approaching or leaving with the different colors as well.
When on the water, that is an awesome feature.