I think SPR sells them so he may be able to give an opinion.
My personal view is that for Ribbing in the UK, they are not much cheaper (if at all) than a proper PLB, which is probably a much quicker way of alerting HMCG if required; and rely on a third party contacting the CG which is a weak point in the system. I'd guess that a lot (but admitedly not all) of the time you'll either have mobile coverage (bear in mind that a mobile will work for 999 calls on any network it can find) or VHF to the CG. In that sense I'm not even sold on the need for a PLB on a RIB, although admittedly your going further than the average person does.
For the point of view of keeping in touch with "base" then if you have mobile coverage then the low cost trackers seem like a good way to do it - and will provide you with a security tracker before/after the trip. I think its possible to do something similar with an app on iphones and similar which lets people find you.
The SPOT won't give you the ability to send anything other than a "good / bad" type message and your location - so I suspect you'll still end up calling in regularly or if any minor problem develops so you can update them. A couple of different SIM cards might be useful for that (personally I find that Vodaphone work best on the West Coast of Scotland, but since orange and t-mobile are now sharing masts it may make sense to add them to the mix).
I'm not against SPOT, a friend of mine took one on a round the world trecking trip last year, and I think it was generally a great reassurance to those back home. Bear in mind however that in the 24 hour news age people back home may be aware of something that causes concern which you are not (in his case he was in Chile during the big quake, he was quite far from the epicentre so didn't appreciate the magnitude, and had gone out for the day leaving SPOT in his hotel, it was about 18 hrs after the quake that he got back discovered loads of concerned messages - in this case by not knowing there was a concern and not having SPOT with him he left those back at home with heightened concern, thinking if everything was OK surely he'd have pressed the "OK" button).