The guitar evolved before the piano. A piano has black keys, which was not always the case in the past. To compesate for the #s and bs, since there were no black keys, the church modes were developed. On a guitar there are frets and the black keys are there along with the white keys only the guitar usually has position markers on the top of the neck and on the fret board. Both instruments are a band in a box that can play the three fundamentals of music which are Melody, harmony and rhythm. What are the differences in playing them and why and how do you adjust to playing them?
The biggest thing that comes to mind is range of notes and theory.
You need complete theory to read piano music as your left hand will be playing from a staff written in bass clef, as opposed to treble clef which is what guitar music is notated in.
So to my mind – learning piano first is advantageous in that you get complete theory under your belt and then you can pretty much pick up any instrument- figure out where the notes are on that instrument and (with practice) master it.