I think I know the answer I am going to get, given that this is "RIBnet" but here goes anyway

apologies in advance if any of these are dumb questions as I know next to nothing about boats, some guys on the LR4x4.com forum pointed me in this direction so you can blame them!
I don't have any sort of boat at the moment but am looking to get something soon. I've had a search around and read a number of the threads on buying but it doesn't really answer the question that is occupying my mind at the moment.
I live in the Falkland Islands and second-hand boats of any sort here are relatively rare so there's not a lot of choice, and freighting something down from the UK is quite expensive. My possible choices at the moment are a fairly old 5.8m speedboat - I don't know exactly how old and nor does the owner but it's got a 1990 Poole harbour dues sticker on it so presumably at least that old - or possibly a 3-4 year old RIB, though I haven't caught up with the owner yet.
Knowing nothing about boats I don't know what "type" the speedboat is, it was registered as the Blue Chip in Poole (its still on the stern albeit a bit faded) and it also has "Wildcat" on it which may be the type? It's got a 140hp Evinrude 2 stroke outboard as the main engine, plus a small aux outboard.
The RIB that may be available is a Humber Destroyer 5.8m with a 115hp outboard, Mariner 2 stroke I think (haven't seen it yet though) so likely to be in good condition so I am told, with a price that reflects that.
The speedboat is somewhat cheaper than the RIB, but obviously a lot older and that is my concern.
So if I were to buy something of this age, am I likely to be inheriting a heap of trouble in a couple of years time with structural problems with the fibreglass? I know this is RIBnet but I figured that the same sort of rules apply to a RIB hull as a speedboat hull in terms of useful life and structural soundness.
It seems to go well enough and the engine runs well (its not the original engine because the original apparently lost an argument with a reef a few years ago), zips along at about 40 which from what I have read is the sort of performance I might expect to get from the RIB so there's not much in it from that point of view. To my totally inexperienced eye it seems mostly OK, there's some minor damage on the keel but nothing that looks serious, and it "feels" solid enough thumping over waves - i.e. it doesn't feel structurally unsound or anything nasty. It's certainly not had any damage "covered up" recently - the hull is in quite serious need of a bit of paint but hasn't had anything done to it for some time.
My preference would be for a RIB if all things (especially price) were equal - the whole wind in the hair experience, been on one a few times and totally hooked, whereas the speedboat has a full (albeit home made) cabin, but with the climate here being unpredictable that's not a bad thing to have. Basically I'm looking for something to mess around with on fine days, serious seagoing ability is not really required (unless I get really hooked) though if it was, I think the speedboat would be more suitable.
So, would I be better to spend the extra (which I don't really have at the moment) and get the newer RIB, or is the speedboat likely to be a safe bet on the basis that if nothing obviously looks wrong, then probably nothing is wrong?
Any help, suggestions or advice on what to poke/prod at would be much appreciated! It isn't possible to get any sort of survey done because I don't think anybody round here does that sort of thing.
Many thanks
Stephen