Depends what your strengths and weaknesses are.
I haven't sat the MCAT but the general consensus is that GAMSAT is more a test of logical reasoning (even in the science section, such that you can't really 'study' for the GAMSAT unless you lack 1st yr university level knowledge in chem/bio/physics), while the MCAT is more a test of your knowledge (which means you can study for it).
A good way to judge the difficulty of the two tests would be to have a cohort of candidates who have sat both, and compare their percentile ranking in each test. My guess would be that the same candidate may score a lower percentile rank in MCAT (ie harder), mostly due to the tougher competition for med school spots in North America.
However not many people have sat both, so we may never know for sure.. 🙂