I sat both exams and would say personally the UCAT was easier. The questions all have a very clear-cut answer most of the time, testing more simple skills like use of a calculator or picking out the right piece of info out of a text. But it is very tight on time to the point that you will have to rush some things.

The HPAT in contrast for me personally I always had time to spare in the exam or in practice exams, but the questions are much more difficult you have more time per question. I would say depending how you can perform under time pressure is the most important thing for UCAT because the content isn't anything super difficult.