Base on your setup, OP, at the end of 15 years (August 2028), if you select the 30 year at 4.5% with an extra $500 per month, you will have paid $134,104 in interest, and have a remaining loan balance of $70,494. If you keep the same payment plan, you'll be paid off in October 2031, paying $139,306 in interest.

If you select the 15 year at 3.7, in August 2028 you're done and will have paid $91,361 in interest. It's about a $48,000 difference. The payment on the 15 year is about $150 more than the payment on the 30-year + $500.