gld/pld is amazingly fun but u need to have ran the dungeons a few times to master the mechanics, i.e. if u do the main story on gld/pld and just to the fights once, you wont be familiat with it, therefore you wont be dong the best you could, and hence you wont enjoy it as much.

logically i advise lvling thm/blm via main story quests and, upon unlocking the dungeons, use gld/pld to run it, and level up using dungeons only. it may sound 'grindy' but trust me its heaps of fun, and i dont agree with the initial rush being over. im in the process of lvling gld and mrd now and run dungeons only. i literally get instant queues, and every run there was someone new to them.

good luck!