For the record, mandolin isn't really harder than guitar, exactly. Well, it may be physically harder if you don't play anything, because those double strings can be like little razors to un-calloused fingers. But actually learning scales on mando is easier for sure, because the patterns stay the same up and down the neck. The four mandolin strings are EADG, same as guitar, but in reverse and missing the B and high E (Guitar - EBGDAE). But not having those two top strings, B and E means you don't have that weird, staggered jump between B and G that guitar has. I played guitar for years, and just strummed chords and could never quite grasp scales and lead. But then when I started playing mandolin and learned the easier to digest scales there, and suddenly it all clicked. THEN I was able to go back and finally learn guitar scales.
All that said, OP, play guitar to start. You'll have more room to learn chords, and it'll be easier to sing along. But pick up mandolin too way down the road when you're solid on guitar.