Do you guys have a planned area where you are going? Panama City would be the best or near Zona Libre (Free Zone) in Colon. In David, and the surrounding areas, you will find a greater population of English speaking since a great many retired Military and Zonians have pitched their tents there. You will also find some really great US style places to eat and shop rather than the tourist trap type of places.
Another thing about David, while being a scenic trip from Panama City it literally is about 7 - 9 hours of intense driving from Panama City not for the weak hearted common US driver. I've made the trip several times but that was after being in the country a year or so and understanding the people's driving style. You will find a great many Panamanians truly believe "blind curves are ideal for passing." It's only an hour or so by plane from Panama City to David.
Driving to Colon only takes about an hour or an hour and a half, depending on the traffic and the number of accidents you could (I didn't say would) encounter. The drive is generally just enough of Panama driving for most people from the US to understand the different driving style there is in Panama. Plus the road side scenery can be rather appealing to the eyes, especially in the morning when the women are all dressed up to go to work, ay carumba...
If you guys are not fluent Spanish speaking I also would suggest paying a translator because while all the tour books state English is spoken by most none of the locals I ever saw spoke much English. It was just easier to speak Spanish rather than trying comprehend some of the broken English I heard, which I found trying to understand broken English more difficult than just speaking Spanish and communicating the easy way.