I've had a common snapping turtle for about a year, and I've been keeping both Duckweed & "Moss Balls" (sorry, don't know true name) with him for some time now. I recently re-tanked him, and have added frogbait(/frogbit?) to the setup - I love all three plants & would recommend any/all of them for your setup. They're easy to grow, low-light (I have a 30" T5 36-W strip light on a 40g Breeder tank and they're growing just fine) and I don't have to fertilize because of waste from the turtle (not sure if you'd have to supplement fertilizer or not - I'd doubt it, but my setup has a massive bioload so I just don't know about a "normal" tank).
In my case, the plants also provide a lot of function for the tank:
-"natural" look that some find very appealing
-additional filtration
-duckweed/frogbait provide surface cover
-moss balls end up losing their shape and provide free-floating bottom cover
-all of the plants provide a constant source of food for herbivores/omnivores
The only two downfalls in my eyes are both maintenance issues:
-constantly (several times daily for me) clearing the filter intakes of plant debris
-constantly removing excess duckweed from the tank & I find it EVERYWHERE (bottom of my shoes, bathroom sink, shower, stuck in my arm hair & clothes... EVERYWHERE)
Basically, to boil it all down to a sentence: I'd highly recommend any of these plants assuming you've done the research and know what you're signing up for.
Hope this info helps you & best of luck with the setup!
Matt