Now that prasugrel is approved which thienopyridine should we use? Is new better?

According to this CCJM review greater efficacy must be balanced against increased bleeding risk, and the decision depends on individual patient characteristics:

Compared with clopidogrel, prasugrel is more potent, faster in onset, and more consistent in inhibiting platelets.

Prasugrel should be avoided in patients at higher risk of bleeding, including those with a history of stroke or transient ischemic attack, those age 75 or older, or those who weigh less than 60 kg.