I use both, for different characters and different stories I want to tell with my characters. They are both great mods. Here's how I see the differences:
Realistic Leveling. Realistic Leveling is a great, easy-to-use, character leveling mod that does not attempt to do anything too fancy or too complex. If you are not interested in getting deep into the nuts and bolts of leveling and only want a "plug-and-play" leveling mod, this is the one. If you want to play "pure" archetypes or uncomplicated hybrid characters, this is the one.
NGCD. If you like to like to micro-manage the parameters of leveling or if you play eccentric characters, nGCD might be a better fit. NGCD gives you absolutely unparalleled control over every aspect of leveling, via three detailed .ini files. You can literally make this mod do anything for you.
If you play oddball characters (like I often do) nGCD is a blessing. For instance, I once played a bitter, anti-social fur trader/scavenger who spent her entire game collecting pelts and other items to sell to merchants across Cyrodiil. I wanted this character to do a ton of bartering but I didn't want her Personality to go up. This would have hard (if not impossible) to do with Realistic Leveling but it was accomplished easily with nCGD.
Also, I use Fundament in about half my games which can affect my decision as well. I always use Willful Resistance when I play a mage. Willful Resistance requires Fundament so when I play mages I often go with nGCD.