MCCode (more often pirated than not) is the most common for people to download and try to learn from
GRPG is probably more often used for actually making a game, most people I know of starting games play the likes of MD/MC/TMU, etc, and GRPG is more like these than MCCode
I always liked GRPG for its simplicity, if there's one thing I hate, it is unnecessary overcomplicating of code! GRPG is for the most part, simple and with a bit of work can be quite effective with bare minimum server load. It's obviously pretty much completely redundant at this point though with mysql and many other functions being binned