After 14 years of service, 50 percent disability, lower back injuries (P2) and bad knees, should I get out or stay in?

Can you do your job? NO- then leave, if you feel you have nothing to offer- then leave. MedB is not always friendly.

Unless I am mistaken, medical retirement does not come with a pension.
Your body may be broken, but your MIND isn't. Leave the heavy lifting to the young, but-- unless you really hate your job (which you've stuck with for 14 years already so likely not the case)-- it is worth it to stick it out to 20 for the pension assuming there is valuable work you could do. Instructors are valuable, senior enlisted advisors (at regular units or at ROTC or other training-type institutions) are valuable... There are a LOT of things you can still do. They might not be 'career enhancing' but the billets need to be filled and you could be doing the entire community a favor by jumping on the not-career-enhancing but important billets out of physical necessity thereby freeing up someone who perhaps had better luck physically to continue to climb the ladder (both literally and figuratively, I guess.)

Don't get me wrong-- I'm not saying to stick around JUST for the pension, but it is a LOT to walk away from just because your body is too jacked up to run with the young people anymore. (I gave up running with the 'young people' when I was in my mid-20's when we discovered that arthritis runs in my family... One of the main reasons I commissioned in the Navy as opposed to staying in the Marine Corps because-- while steel decks still suck for you-- the Navy doesn't quite have the 'culture of fitness' that the Marines do. I wanted to be able to make it to 20, and I knew my body would never last that long in the Marines... I had already permanently damaged both ankles, both knees, and my shooting wrist in only 3 years, so I chose a less physically demanding-- but more mentally challenging-- path in order to be able to do so.)

I have a neighbor in a similar boat to yours (it sounds like). He's an E-7 Navy diver who can no longer dive because Navy dental accidentally smashed his face during what was supposed to be a routine surgery. He had about 17 years in at the time. Just because he can't legally go in the water anymore, he still supervises and advises the junior divers. His LEADERSHIP is still valued despite the fact that he can no longer engage in his particular physical area of expertise. It could be unique and just his good luck that diving is a field where there is ALWAYS a supervisor on the pier, but you get the gist of what I am saying.

It's your body brother, it's not easy to make a decision like that, I'm facing something similar. Just had my second shoulder surgery and have to wait and see how things turn out. The Army will go on without both of us. A medical retirement is not a bad ending, albeit not the ideal ending. But at least you seem to have the choice, whereas many others end up without the choice.