I'm a 3rd semester RN student in an AA program. I wondered the difference in difficulty level as well. I can tell you this much.. they are completely different. From a very general perspective, science courses are all about memorization. Whereas, nursing is all about critical thinking.
The science courses did feel a bit more demanding in learning new material at first, however, most students will learn to adapt to the studying.
In nursing school, you need to learn a lot of facts (e.g. lab value ranges), and then interpret them into what they mean. Furthermore, you need to learn to look out for signs and symptoms that can be expected from any abnormalities.
In my experience thus far, nursing school isn't as bad as I thought it would be. It is completely doable, it just take a little more to adapt to its own nature of implementation. Science courses on the other hand are definately challenging, but when all said and done, it's basically memorization. Science course though are definately hardcore learning, whereas nursing is merely implementing (not as easy as it sounds). I know a lot of students who ACE the exams, but do poorly in clinical.
TIP: If you get into nursing school, get into the role. I've seen students (myself also) talk talk talk about achieving something more. "Nursing is just for now." You will look like an idiot time after time when your ego starts to spill out of your mouth like this. Enjoy what you do as a student nurse and learn as much as you can. If you end up in a med program, rad tech, whatever, you'll have that extra perspective of patient care which can go a LONG way. In addition, lets say you were in med school, while everyone is wondering, what's cirrhosis? You'll already have a head start in the game.
Enjoy your life!