I would think so. It's a lot harder tempo, generally, which demands technique. And as the Nocturne is slow and heavy, the waltz is quick and light. It's actually sort of two oposites, I think. Though I would consider the Waltz to be harder.
Well actually definately now that I listen to it. The waltz also contains a set of runs which could be demanding. Not to mention the cadenza-like part in the Coda. Yes, I would put the Waltz in Em higher on the food-chain.
Oh, but I agree with mr man over there. I would definately play them both. Because they're so different you'd be challenging yourself in more at the same time, pluss you get two new great pieces on your repertory. (I don't really know the word in english, it just felt natural)..
Though, I find the 'musical' demands in Chopin's Nocturnes to be...well...quite easy. At least compared to certain others. As someone said, Chopin is a sort of.. Romantic for beginners. It's quite simple to understand what he's after. 'Musically' compared to Liszt or Schumann, he's easier, I think. Not that there's anything bad with that, just that I don't find the music in the Nocturnes such a challenge as his runs.