I solved the megaminx by myself the same day i got it using slightly altered 3x3 algs, the square-1 i got into cube shape after loads of work, but pieces were messed up, personally i think that the megaminx is pretty much a time consuming 3x3, while the square-1 is more of a horrible mutation of the 3x3, it looks pretty simple at first glance, but after a few turns its there laughing at you, the megaminx just look challenging, but is pretty easy to solve
I got really tired of my sq-1 because my average won't get sub-50. And the megaminx takes me like 3 minutes to solve, I get really bored in the middle of megaminx solves...It's basically like a 3x3 that takes way longer to solve.
Go for the square-1.