I've tried both. Megamax is very similar to the ABU Simple Ultra and PeekABU diapers. The will easily hold up for 10-12 hours, and really are great daytime diapers. After that they threaten to leak from around the back of the legs, though leaks do tend to be smaller. They do have lower tapes, but they are positioned a little too high. I found you need to place them below the tape landing zone, and have them angled down in order to work correctly. They do provide a decent snug fit around the legs this way, but beware they will be permanently stuck and cannot be repositioned or removed without destroying the shell.
In comparison, betterdry (or crinklz) will hold up for 16+ hours. After that leaks can occur from either the legs or waist, which also tend to be smaller leaks at first. The shell does feel thin but always holds up for me just fine, even with active wear. However, the shell will rip if after 12 hours of wear you try to retighten the tapes. (Which will further stretch the shell). So as long as you get a good fit with the tapes on the first try then one diaper can last a full day, even with staying well hydrated.
I also found the mega max diapers are NOT good for my overnight side sleeping. They will leak on me every single night, in spite of changi g just before bed. This is because of the slightly lower sap content and less front side padding as compared to betterdry. Though betterdry will still slightly leak on me about once a week too I have found them to be the best currently available.