Mecha is methylated, so a bit harsher on the liver in that respect, but will not aromatize so no gyno worries. TZ can start or worsen gyno in some people, so you need to be prepared for that. On the other hand, it's not methylated.
Either one is a very solid product. You just need to know how to use them properly to maximize gains and keep negative sides at a minimum.
BBG is right about TZ...if you take enough of it, it could pack more size on than Mecha.