I saw MP in London, though it's virtually unchanged in the transfer. Saw Little Mermaid in August.
Mary Poppins has one of the most magical sets ever. And ther'es a major surprise at the end that makes it really incredible (won't spoil it!).It' a show of a lot of technical wonders, and I find a lot of music to be beautiful. It's definitely a show of spectacle. Little Mermaid still has that sense of spectacle, but in a different, more cartoonlike kind of way. I really enjoyed Little Mermaid, but I wouldn't call it as good a show technically. Both are really good. I would say it varies more on your kid's interests.
Sidenote- My little sister fell HEAD OVER HEELS for Flounder when we saw Little Mermaid. She still talks about him, as it's played by a kid. And there's two child actors in MP too... So I can't decide! But they're both very excellent.