As these sections draw upon a general aptitude for problem solving, learners inclined to logic puzzles over scientific knowledge would find them more conducive to their career path.
In terms of academic choices, the BMAT is a requirement for much fewer universities although the list includes Oxford, Cambridge, Imperial and Keele. If Oxford or Cambridge are on your list then there is no alternative to BMAT.
In contrast, UCAT is accepted in over 30 universities across the UK and its examination dates range from July to September which means results are available prior to university application. The BMAT examination, however, is taken after UCAS university applications have been made and as such poses a greater risk if a sufficient mark is not achieved.
As you can see, personal skills and choices are the deciding factors in whether the BMAT is a harder test than the UCAT. It is best to test yourself with sample papers from both to judge your aptitude before making the choice that is best for you.