IIRC AF was the backbone and inspiration for the Natural grow mods both which RL and nGCD are part of.
To clear up the history there...
Balor's Leveling Mod for Morrowind was the original. It was a simple smooth-advancement system; IIRC it was set up so that every 3 skill advances gave you 1 point in the related attribute. There may have been some variance because some attributes had more or less associated skills. Leveling was basically vanilla other than that, I don't believe it even had a retroactive health system.
Galsiah's Character Development (GCD) took the basic idea of Balor's, and greatly extended it to create a completely new scheme for advancement. It included retroactive health; alternate magicka calculation (and optional, very slow real-time magicka regeneration, which Morrowind and previous TES games didn't have); attributes over 100, a global slow-down system where at high levels, it was harder to raise low-ranked skills; and each skill had an effect on more than one attribute, plus might increase your magicka and/or health independent of increases to Intelligence and Endurance.
Galsiah knew long before Oblivion was released that he would not be moving on to this game. I really couldn't bear to play the vanilla system after GCD, so I took up the task; nGCD is "not Galsiah's Character Development," named after his mod. As this happened before release, I believe nGCD was the very first leveling mod
proposed for Oblivion, though it wasn't the first to reach a playable state. (Early versions were rather terrible, and not just because I had made several invalid assumptions based on Morrowind mechanics.)
I believe the first released and fully-functional Oblivion leveling mod was actually a direct port of Balor's. Beyond that, Kobu started work on his mod shortly after the game came out. (At one point he asked me to make my mod an alternate-defaults version of his, but I wanted to bring in some GCD features that Kobu's framework simply didn't support.) AF's mod was also started somewhere in that time frame; I'm pretty sure Kobu's was released first, but it might have been AF. nGCD was in active development this whole time, and actually playable around the same time as Kobu's.
Really, IMO the reason Kobu's became the most popular was simply that he cared about marketing it! nGCD was always mainly written for myself, and I didn't mind some roughness in the UI and documentation; Kobu put extra effort into polish, accessibility, and a good user's manual.
nGCD has grown more refined over time, of course.
Realistic Leveling showed up at some point during one of my vacations from the Oblivion scene. I actually asked ABO about it, and he said he wasn't even aware of nGCD when he made it! So the fact that they are similar in many ways is actually kind of neat.