Bishop or FP, I/L is for fun only. Don't be like me and waste your time on one. You will find yourself having to pump so much meso into being able to 1HKO petri, which could otherwise going into a HS + Leech in one character [bishop], or lower funds to 1HKO petri with f/p's.
F/P is harder to lvl at 2nd job, easiest to lvl at 3rd job (a bit slower than chief bandit IMO, but faster than any other class), and cheaper ele wand 5 at 4th job I/L is easier to lvl at 2rd job, alright at 3rd job, but 4th job ele wand 7 is more expensive. Other than that, they are about the same, they are both able to S>leech at ulu 2 at an earlier lvl You do need quite a bit of funding to 1 hit petri.
imo I/L is the most fun mage there is, the class is easy to level throughout 2nd, 3rd and 4th job as all of them have mobbing skills, chain lightning is nice to mob as well (and I really enjoy the animation and bounce mechanic) and iirc Ifrit has a bit more damage than Elquines. While 3rd job F/P is very fun and among the easiest due to mist training, I find their 2nd job horrible lmao, and Paralyze in 4th being a single-monster skill kinda doesn't do it for me haha However, I/Ls are more expensive to fund than F/Ps as elewand7s cost more than 5/6 ones (check Sylafia's price guide for reference) and have less total magic attack (ele5/6s have base INT while ele7 has LUK only). Also, you're definitely not getting into runs unless you have friends/guildies who are willing to take you in lmao, meanwhile at least F/Ps can join bearwolf runs at LHC due to their fire weakness. Leeching is fine on both of them since Meteor and Blizzard have the same basic attack, but you have to level up a hs mule and duo client with it.