the best nights are going to be Thurs-Sat in both places, so you are going to have to pick which you prefer.
As for quality per percentage of chicas, Jaco wins. Fewer girls over all than the Del Rey but a higher number of what I consider HOT.
Prices are about the same, girls might(probably) will ask for more in Jaco. But can generally be had for 80-100 if you are a decent regular dude. Some casanova types claim cheaper rates

but who knows.
The girls are there to work and make money don't waste their time if you are not interested and don't let them waste yours.
If this is your first trip, nothing written here can prepare you for what you are about to walk into.
Your mind will be blown.
READ all the newbie threads and read some more.
Easy to screw up and blow a c-note on a unsatisfactory experience, but it happens to everyone.
Shake it off and move on.
Be careful who you hit on, remember not every girl walking the beach in Jaco is in play. They will let you know if they are, trust me.
Generally speaking if girls are in Del Rey, Key Largo, Sportsman, Cocal there is a high probability they are working. Monkey Bar in Jaco can be a mixed crowd, pay attention and you will see lots of girls from Cocal there later in the evening, rates can be cheaper there the later is gets.
Keep a positive attitude and enjoy yourself, I find it damn near impossible to not have a good time in Costa Rica.
You will not believe the quality of women that are available, it is absolutely crazy. You will have a great time! Geez, writing all this makes me jones bad, wish I was going with you!!