L.F.
You shouldn't have to choose between two evils.
You know you can buy a prepaid phone at Walmart for about $20, right? Just buy a cheap T-mobile prepaid phone from Walmart, Target, Best Buy, etc. Then you can either call T-mobile or go to your account online to activate your phone to your existing account. It probably won't have a nice camera, but it will get you by until Christmas. If you have any problems with the phone, you should have at least 15 days to return it for a full refund.
You should also try taking the LG Optimus to T-mobile to see if they can give you a new battery or just replace the phone for free.