Playa Bonita is super close to Panama City. This makes it easy to cruise over and spend all your time enjoying the beach and resort that is here (I think there are only 1 or 2 hotels, but they shoudl be all-inclusive). However, I don't think that the beach is as nice as Playa Blanca, given how close it is to Panama City.
Playa Blanca is about 2 hours from Panama City. You'll spend a bit more time on the road, but the interstate here is smooth and quick. In my opinion, it's probably worth the time spent getting here, as the beach is bigger and nicer, and you'll have more options for hotels and activities. Either way, you're going to have fun!