So id say BLM 1-50, RDM 50-70. BLM starts out fairly simple(fire stance for damage, ice stance for mana regen, keep thunder dot going whenever possible). It gets its staple AoE attack early(fire 2 at level 18), so dungeons arent too bad. And you really only have to hit one or two buttons every 2 seconds to do good damage. Dodging attacks while trying to finish casting can be annoying, but you just need to learn attack priority(this attack will kill me so i stop casting and run, that attack will hurt me so i can finish casting then run, some other attack does very low damage so i can ignore it and keep going unless my healer is dead or mad at me). At higher levels(over 50) BLM gets more complex as you have more stuff to keep track of, which is where you could potentially switch to RDM(which you have to be 50 to unlock anyway). RDM is also fairly easy(cast quick spells to proc duelcast, use duel on long spells, balance mana levels, use melee combos when up), and while the damage is lower it has a couple advantages. Its much more mobile then BLM(so dodging is less of an issue), and at higher levels it has a heal and a res that it can bust out in emergencies(BLM has essentially no unique utility currently, just damage).

Best of luck!