It is a very interesting question. I agree the cabins on the (Non-Epic) NCL ships are well-appointed; they have nice bathrooms, coffee-makers (which is ideal for me) and usually a nice desk area.
The Carnival staterooms are probably bigger, but they are more spartan. I do like ones on the Spirit class, however.
I think the entertainment on the bigger Carnival ships is really good, but I also like NCL entertainment and I think it is a bit more sophisticated (no dancing waiters, etc.). The decor on both is pretty "whimsical".
The big difference is the dining. carnival has better included food in general, by far. You can get almost anything you want at no extra cost. But NCL has better alternative dining options, so you will probably find better food, but at a price.
In the end, I think I would just go with the one that offers the best deal, but I would always stick to the newer ships (I prefer them over the dated-looking older ones).