The most recent person I know who joined the Army as a linguist (35W) is getting $39,000 bonus, plus $71,000 in college money.
Even 8 years ago, when I joined, the linguists were getting $20,000.
I know a lot of linguists in the Army,
it is not unusual for a linguist in the Army to be promoted to Sergeant First Class (E-7) in 8 years.
Half of the time that you are in the military as a linguist, you are working together with all of the branches, so join the branch that suits YOUR needs best. You also want to look at promotion opportunities, which are usually best in the Army.
No matter which branch you choose, before you go to MEPS to sign the contract, have your recruiter put you in contact with an actual linguist, so you can get the real deal from someone who knows.