Thought I would add my recent experiences , hope it helps.
On my 6.2 metre RIB I have a GPS system and a conventional compass. I tend to use the GPS for Navigation, and the compass for general headings. When in any rough sea the conventional does jump around a bit, and isnt really practical for exacting navigation - wouldnt want to rely on it to get me to within 10 degrees or so over any distance However, forgeneral bearings - o in power failure situations - I wouldnt be without it.
On my small 4 metre RIB I only have a digital cmpass, which nicely gives out readings, but doesnt tell you general drections.
I guess given the choice of one or the other depend what you want to do - how far and in what sort of conditions.
Guess thats not really a lot of help but perhaps something to think about.