If you can only play one, first decide what your end goal is. What do you want to be doing musically a year from now?
Do you just want to enterain yourself, play only at home? Then maybe a guitar is best. Nobody really ever sits down at the campfire with friends, pulls out a bass, and sings along. Bass often doesn't really work as a solo instrument, or as a singers only accompaniment. Only bass players really appriciate the sound of bass alone. Typically friends and family don't get it when they hear you play bass alone.
Do you want to be in a band? As others have noted, the supply of guitarists is pretty saturated. Ya can't fling a dead guitarist without hitting another guitarist. The place is crawling with them. Bands need good bass players. There are already too many guitars. If you play bass you are more likely to find a spot in a band than if you are a guitar player. Bass sounds GREAT in a group! I think there is more room for doing more new creative stuff on bass. I kinda think everything you can do with a guitar has been done.