The similarities:
Both were found guilty of serious crimes.
Both men alleged they were not guilty. Mann has protested his innocence throughout, blaming (among others) Sir Mark Thatcher for the conspiracy. Despite Gromit’s assertion Al Megrahi was not “innocent”. He may have protested his innocence, but many people do. He was convicted in a properly constituted court and abandoned his appeal against that conviction. The question of his guilt or otherwise played no part in the decision on his release.
Both men are said to be unwell. However, apart from needing medical treatment from time to time neither is “dying” (any more than anybody else is). Al Megrahi has done an “Ernest Saunders” and recently recovered sufficiently from his illness be allowed home. Last July the same illness was forecast to finish him off within three months and this was the reason given for his (considerably) premature release.
The release of both men was “nothing to do with the UK government”. Al Megrahi was released by order of the Scottish Justice Secretary Kenneth MacAskill, and Mann probably on the say so of Equatorial Guinea’s President Obiang Nguema. I cannot imagine that anybody in the current government has worked too tirelessly to secure the release of Mann (Eton, Sandhurst, and the Scots Guards).
The differences:
As far as I know, Mann was not directly responsible for any deaths, Al Megrahi for 270 civilians, some of them on the ground in Scotland.
Al Megrahi was convicted and sentenced by a Scottish court whereas Mann was convicted and sentenced by a court in a country which, to say the least, is not renowned for the impartiality of its judiciary.
So my answer to your question, jake, is that if there was an overwhelming case to release Al Megrahi, there is an ever greater case for the release of Mann.