Having read and Ref'ed Prison Planet and own Beltstrike. Each have their strengths and weakness.
Beltstrike seems to be best used in fragments of a larger Campaign with no reason why players should keep coming back to the system other than if they were Belters (but then what would be the motivation for them leaving the system???). For it does contain excellent mining rules but as it has been said it adds flavour not substance unless you game is going to be centred around a Belter Campaign. I have found the adventure to be rather unsophisicated compared to Tripwire but that is naturally as Tripwire is a campaign. It is very good as a generic SF adventure but as for it being a Traveller adventure...it certainly is not OTU which it makes no claims to be. So, if you want OTU stick with the original Beltstrike and embellish it with the Mongoose product and you will have a winner.
Prison Planet is better written but sort of leaves the players hanging. I found it work to motivate them to be other than "good", so again best played within an existing campaign or provide solid motivations for escaping the prison. It does drag in parts but still feels true enough to the OTU. Could it be used without the OTU. Certainly, but there is enough of a Traveller feel to make it work.
Tripwire - to anticipate the question of some...the parts that worked worked very well. The parts, in which, you had to embellish failed. It is only nominally set in the OTU, as we have known it, to date. I don't want to be too harsh as clearly Mongoose is the process of re-booting the entire OTU and that process sometimes means that we have give up some of cherished notions/things for the greater good. So, when they release Secret of the Ancients Campaign later this year we might get a better sense of their version of the OTU.