My opinion...LEARN PIANO FIRST OR AT THE SAME TIME!
Pianos have weighted keys, organs don't. Organs can hold their sound forever and a day, pianos can't. Therefore, organs can make you lazy (exactly what it did to me) and playing piano becomes hard and your arms will hurt. Piano takes stregnth to play. Me, since I SOMEWHAT bypassed piano, I like organ better and I have a keyboard at home since I don't have either an organ or piano. I like organs better than pianos because you don't have to beat it to death and as I said, you can be lazy. but if you don't ever plan on playing piano, you don't have to, but i suggest you do.