Definitely a bed! Folks are used to a small studio with just a bed, and I agree with @Pat271 that most sofa beds are (a) uncomfortable and (b) a hassle to set up! Folks would be inclined to just leave it as a bed for that reason - which defeats the purpose of the sofa bed.
Lastly, many Millenials and younger guests hang out on the bed to watch shows, etc., so unless they need a desk for work, there’s no problem there. There are some great fold-down desk options, and perhaps even a good -quality folding chair to go with?
All that being said, I almost bought an excellent sofa-bed 3-4 years ago: it was 3-4k then, and would have taken several weeks to arrive; company in Texas (don’t remember the name) if you decide to go that route.
Best of luck, and keep up with the AirBnb forums as you will get lots of great suggestions and advice!