Leves are good, but they are finite, while collectibles are not. Leves are best for ALC. I did not even touch his collectibles. All other classes I leveled with collectibles mostly. Plus moogles quests for additional exp.
GC turn-ins I did only for crafts with * (double exp). Of course you should try craft HQ for x4 exp.

I leveled all to 60 in 2 months. And made 250 millions gils in another 2