Interbus is a great option for travel within Costa Rica.  To Jaco, it costs $19 per person each way.  Door-to-door service, they pick you up at your hotel and drop you off at your Jaco hotel.
They use 8-person mini-buses with A/C.  But they are never full, lots of room and very comfortable.  They make 1 rest stop but they can stop for you to use a restroom whenever you want.  Drivers usually speak English.
A personal driver is cool, but it'll cost you $70+ each way, especially 2 days apart.
www.interbusonline.com
Note: 
You must book them at least 3-4 days in advance because you have guarantee your reservation with a credit card.