I also learned it both (and now I'm a organ teacher for my church, but still learning piano)
Well, the main difference of piano and organ is...
on piano, we playing with articulation and dynamics. But on organ, dynamics and articulation are 'OFF'. We just play it on same and stagnant dynamic, and dominated by continuous sound, like chord.
Of course, learning organ is easier than piano. But note if you're now on piano, very easy to learn organ, because we can bring our piano basic technique to play organ. I just need 4-5 years to learn all of organ technique, and now I'm a senior member of church organist)
But, of you learning organ first, it will be very difficult to learn piano, since 2 of my students on organ want to learn classical piano, EVEN their fingers can't make continuous appergio smoothly, like stacatto (I order a simple test, to play key C-D-E-F-G, but they hit those keys one by one!). Seemed that they lack on basic technique.