DKCR Returns only required memorization for two or three boss fights and the mine cart and rocket barrel levels as far as I can think of (currently 180% done with the game). SMB (almost completed that and also beat the game) does have some a handful of levels I can think of that require some memorization too. DKCR has more than SMB, and it's a valid complaint for both games, but I'd hardly call it a game breaker. I think the levels are a lot of fun, so dying and remembering where/how I died didn't really bother me. SMB was the same way for me.
All in all, SMB had incredible platforming. I prefer DKCR more (which also has fantastic platforming) though, simply because of the production values, set pieces, music, the great way that levels unfold, etc. Both are great games, and I'm kind of surprised that people would hate on either, but to each their own.
Biggest complaint is that the level design is GREAT for speed running, but the camera doesn't pan enough for it since the roll jump is so quick. Therefore, if you want to speed run a level then you've gotta memorize it. That takes the fun of speed running out of the levels, which is sorely disappointing, as I LOVE speed running and SMB really hit a perfect sweet spot for me there. Speed running the original DKC games and classic mario games are much more fair since you can't accidentally go flying into unseen enemies/pits at high speed so easily.