Hmm. Well I did both- I did a one year masters program and then I taught for a year. Both were good experiences.
I think the masters program helped me a lot more than my experience teaching (in terms of getting into med school). But I think it depends on the person. How's your gpa & mcat score? Will a good masters gpa make up for an average undergrad gpa? How much do you like research? Make sure that the masters degree will be beneficial before you blow a lot of money on it. For me, I loved my masters program and it was worth every penny (I studied repro bio at the hopkins school of public health). And the program did exactly what I wanted it to do. I was an engineer in undergrad and the masters program gave me more exposure to medicine, biology, and public health. But also keep in mind that masters programs help you way more if you've *finished* the program before you apply. Otherwise you've barely done anything in program... so what exactly are you showing the AdCom that makes you any better than everyone else?
Now teaching is a lot of fun and will be really helpful, especially if you're interested in peds. Teaching shows that you can handle little kids, that you're patient, that you are comfortable in front of an audience, that you have the ability to make concepts simple and teach them to someone else, that you're diplomatic enough to deal with really annoying parents. All of these are great skills for a future doctor to have. But ask yourself: How much do you like kids? I didn't do teach for america. Instead I taught sixth grade math at a private school. The kids were awesome and I really loved teaching. But teachers do have their own problems... you'll probably be working in a public school... so you'll have kids with behavioral problems. And parents complain about everything (but then again private school parents are probably more anal that public school parents). Another good thing about working/teaching was that I made some money. I don't know how much teach for america pays, but starting salaries in this area for public schools is like 35k or something (which isn't bad at all).
Hmm. Wow, sorry, I wrote a lot 😛