If you want a long term career I would pick boxing. It's more lucrative and you can do it for a longer period of time (MMA is more barbaric, lets face it). Boxing is fun, so would be MMA, but the return is not as big imo.
Boxing is the superior sport hence the tens of millions in the difference in comparison in top vs. top
I say do boxing at the same time get your legs involved, your elbows as well. Use whatever you got for striking. Do whatever you enjoy, do both.
Growing up I've always wanted to fight as a career, I'm not a violent person but i just love the two sports. I've tried getting into them a few times but didn't have the dedication i have now. I've had about 3 months of formal boxing training and roughly 5 of TKD. My dad was a boxer at my age and actually won Golden Gloves a few times. I'm really good at striking, i have near perfect form even though i havent been formally trained in close to 2 years. I finally found a gym by me that has Boxing, BJJ, Judo and MMA. I've been naturally good at defending takedowns, most all my friends wrestle and have done it for years and they could never get me down, I actually beat some of them a few times with no grappling training at all. In middle school at our end of season football party we played keep away and i had 8-10 people on me trying to bring me down, i could hear people going like "are you serious, they can't tackle him?", they never did end up getting me, i ended up throwing the ball off to someone else. I like Boxing because it's fast paced and is all striking. I'm interested in Judo as well, i'm fairly strong so i feel it's something i can excell at. My only problem with MMA is that there are alot of fights that end up on the ground for the duration of the fight with someone just grinding out the other to score points. The movie Warrior really makes me want to do MMA but it usually isn't action packed like they make it in movies. What do you guys think?