GoodRx shows you both levels of Medicare co-pay prices as well as other prices you can get when you pay cash, use a discount coupon, or use some other way besides your Medicare coverage.
When you purchase a drug with Medicare, the cost of that drug counts towards your plan's limits (described above). This can be a good thing (getting you out of the donut hole or a deductible stage more quickly), or a bad thing (putting you into the donut hole more quickly).
When you purchase a drug without using your Medicare plan, the cost of that drug will not typically count towards your limits. Some plans will allow you to submit receipts for drugs purchased without Medicare for credit towards your limits though, so check with your plan.
Ultimately, the decision whether to use your Medicare coverage is up to you. You'll want to consider the savings you would receive by not using Medicare, your expected annual drug costs, and if you will spend enough on prescriptions to enter the donut hole or post-donut hole stages. Your Medicare plan can give you more information on your total spending and help guide your decision-making.