The front lever focuses more on the abs and shoulder girdle.
The back lever focuses more on the lower back and shoulder flexibility as well as building bicep tendon strength if done with chin-up style grip (supernated? - i always get this bit the wrong way around).
For planche you need very good shoulder girdle strength, bicep tendon strength, lower back and abs. Plus wrist strength and/or flexibility
Both will help, plus planks and handstands