If you want to compete, you will probably benefit from doing more than just 2 styles. right now i do 4, but they are all connected: BJJ, Wrestling, Muay Thai, and Boxing.
you deffinitely need atleast one grappling and one striking, and like people have said already, MT and BJJ are usually your best bet if you had to pick.
The problem with BJJ is that you wont have many takedowns to choose from, hence why wrestling is a great choice(look at all the great fighters that use it as their base: couture, fitch, Kos, St. Pierre, Hendo, Faber).
MT is great, but due to the use of kicks, punches, knees, and elbows, you normally wont get great at one thing, more good at all of them. That is why boxing is great to give the hands more time, since you can normally survive with good boxing and kick defense(rampage, BJ, Marcus Davis, Arlovski).
I get all these at my gym, so if you can, that is prefect if not, dont be afraid to jump around
Also, having an MMA class that ties everything together is great for getting to know how it is when you have to combine everything.