Depends on what you want to do in the game. If you enjoy going on missions and collecting lots of cards for trade, Energy is where you want to invest. If you want to be able to attack more often, put your points in Attack. If you want to prevent people from winning in their attacks on you, you would want to put more points in defense.
The amount of Attack and Defense points you need will also depend on your deck. Attacking once spends the entire power cost of your attack deck (your five cards with the highest attack values). Defending only spends about ten percent of the power cost of your defense deck (your five cards with the highest defense values). A good rule for the minimum you want in Attack and Defense is enough Attack to use your best attack deck twice, and enough Defense to use your best defense deck a few times.