I am majoring in music in school and have a Performance class. I have to pick either guitar or bass as my instrument(I narrowed down a bunch of others). For the class, you have to play in an ensemble, take lessons, and perform some stuff for the class. I want to pick an instrument and really get to know it inside and out and work on mastering it through school. Here are reasons for each instrument:

Guitar: I like to play melodic stuff and really like to listen to what is being played and how it fits the song emotionally. I like to play slow stuff and relax while playing. I have Pro equipment for my guitar already and like how my set-up sounds. I find it more of a fun instrument to play in most genres and want to be in a band.

Bass: I only have a 20W amp so playing in a band would be difficult until I upgrade. Though I like to play guitar, I really like playing some cool basslines like "Joy Ride" by The Killers. I also like to slap and pop. I find that there aren't as many bassists around and I'm usually the kind of guy that will play whatever and this is how I ended up with my bass.The styles I like to play bass to are kind of dance funk and stuff like that, but I also like metal and pop rock stuff too where the basslines aren't as fun.

Additional stuff: I listen to all kinds of music like Slayer, Metallica, Fall Out Boy, All Time Low, RHCP, Rancid, Bad Religion, etc. I want to start a band and base it off a today's style of music.

What instrument do you think I should keep as my main one(I can give you more info if you need)?
I mean, it's not like you have to stop the other. I chose guitar over bass but I still play bass all the time. I think it'd be cool to see you do bass since not many people do it. And metal and pop rock don't have to be simple and boring, make it cool. The players just so happen to not be that skilled or theoretically informed IMO.
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sounds like your "guitar reasons" can easily be bass reasons...id go with bass man, but then again im biased
figure out which one you would want to make a career of more. which one you can picture yourself on stage or studio or whatever doing. and if youre going to be studying music, i suggest opening ourself up to more styles than rock i.e. jazz, flamenco, big band, classical etc. it really helps not only in theory, but it opens creative doors. as well.

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I have to play in a rock/jazz ensemble too.

I'd rather be playing in an All Time Low style of band than a Devil Wears Prada band and I heavily listen to both. I'd want to be a radio style band that plays simple, yet catchy and fun. I'm not going to stop the other, I just want to focus mainly on one. Then again, I get into moods where I'll want to play bass over guitar and vice versa.
I play both, so I say do whichever you want, if your looking for something fun and simple but still challenging, go for bass, but if you want to be versatile, go for guitar.
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If I were you I'd focus more on guitar since you want to play simple catchy stuff alot, and it will be alot less boring on guitar than bass. But then again theres less people who are really good at bass and you could pretty easily stand out in that crowd compared to the millions of guitarists.
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I dunno man they are both good instruments, the guitar just gets all the hype. although it may be fun to make fun of bassists and such bass isn't as simple as it seems. I mean look at people like Claypool and Geddy. it can be as complicated as you want or as simple as you want.

and bassists (not shitty guitarists that were given a bass) are harder to find than guitarists, because guitar is the popular instrument right now. but I say play whichever one you like. If you end up playing bass just because you're the only person someone could find then you'll never be as good as you would if you actually liked playing bass. and of course that can apply to any other instrument.
I'd say guitar. It seems you like playing guitar, but bass is just when you want to slap or pop (which is something that you shouldn't use too much, since it gets overused way to fast) and you don't really like playing fingerstyle as much. And you say you play bass because its easier to get a gig, which makes it seems that you don't really like it all that much, and its more of something to fall back on when you can't play guitar.
I'd say guitar - if for no other reason than it sounds like you'll get better grades in your class if you play guitar rather than bass, sounds like you play it more.
No reason to stop playing bass in the background though