My experience is that nGCD is geared more towards players who want to play a conventional, dungeons-and-dragons-style "class"...and stick to that class for the duration of a game. Realistic Leveling tends to be geared more toward players who want to play either a jack-of-all trades character, or a character whose skill set changes during the course of a game. I alternate between the two mods regularly, depending on what type of character I plan to play.
That said, both mods are
very configurable, via .ini files.
With either mod I always use http://www.tesnexus.com/downloads/file.php?id=14304 Progress allows the player to have complete control over how fast skills level up. It also offers new ways for skills to level (missed spells add a percentage to advancement, etc).