Why not both? I didn't think they were as hard as, say, Korean or Arabic. I thought they were easy, and maybe you will, too.
If you were to learn both, I'd take Spanish first. If you say a word in Spanish, you can sound it out and figure out how it's spelled. French isn't like that. Also, the French future and conditional tenses are easier to remember if you know Spanish.
Aller (French: to go) --> tu iras (you will go)
Ir is the Spanish verb that means "to go". Look, it's in the French stem.
Être (French: to be) --> elle serait (she would be)
Ser is one of three verbs that mean "to be". The other two are estar and haber, but that's another lesson. Look, ser is in the French stem as well.
There are a few more ways Spanish helps, but that's all that comes to mind right now. If you have to choose one, know where you're going. Spanish is helpful for the southwest US, Spain, Central and South America. French is helpful for, well, France, Belgium, and north-northwest Africa.