Well... you will want to get Thaumaturge to 26 for Swiftcast for all casting jobs and Arcanist gets the very useful Eye for an Eye at 34 so if you want you can level both up for cross-class skills and to see which you like better. I feel that by level 35 both SMN and BLM will have enough of their tools for you to get a good idea how they will play and which you like better.