You only need about a semester of volunteering. Do both and earn the money. If possible, try to observe the opt seeing patients. Also, decide on a path you want to take and focus on it. If you want to do med school an opt job may not be the most relevant.
I vol'd for 50 hours at a hosp ED, waste of time but important for my application. I spent a ton of time doing research and got published 3 times -- which is very lucky. It has helped a lot, though. To do this, check profs on campus for publications using Pubmed. Ones that have a lot, and have 2-3 in the last 2 years are the ones you want to work for. Let them know that you want to get published and they will try to accomodate.
Good luck.