Hi Jim,
Definitely take Highway 34,the coastal highway. It’s quicker, easier, and newer (at least parts of it) than the mountain road. It’s a pleasant drive that should take you about 3.5 hours. Overall the coastal road is in good shape.
The mountain road is actually the PanAmerican Highway but picture a winding road, lots of switchbacks, and drops of thousands of feet. Not necessarily dangerous but that kind of driving can take a lot out of you. And the drive takes about two hours longer than the coastal route. You actually pass by the highest point in Costa Rica on the mountain road – 11,000 ft+.
Jason