I watched both shows during their first run. So did all my friends. I was 12. And trust me, SatAM was the show I was crazy about. I think we all had a love-hate relationship with AoSTH. We all said we liked the ABC show better, sometimes said we hated AoSTH, and wished AoSTH could be SatAM ... but yet, none of us could stop watching AoSTH.
But, still, I wouldn't go so far as to call AoSTH "babyish" by comparison. Yes, SatAM was, in theory, the "serious", "dark" show, but it doesn't hold up too well for me when I revisit it. Now, it seems like ... well, a kids' show. And not much less of a kids' show than AoSTH. If SatAM is the serious, dark show, then AoSTH is the absurdist, trippy show. (Yes, trippy. I'm gonna go with that.)
Also, I don't think it's accurate to say that SatAM had the better animation. SatAM was drawn more realistically (especially the backgrounds). That's a design, not animation, factor. Character poses were reserved on SatAM, more exagerated on AoSTH. But the animation itself wasn't particularly better on either show. Somewhat awkward, stilted, fairly limited animation that was pretty much the norm for what DIC produced for TV during that era. Both shows had their best, most fluid, most dynamic animation during their respective title sequences.