I'm using a Sneak/Assassin character (Daggers+Bow) and it seems that when fighting (after I'm detected) it is worse to use Power attacks than normal attacks.

When I use a power attack (have to be in range) it seems that it leaves me open to get hit a lot and doesn't seem to be worth the damage.

Am I doing something wrong or is it better with higher damage weapons (or shield)?

It is better with a higher damaged weapon.

I play the same type of character as you, and I've found that using Unrelenting Force to stun them is a good time to power attack. Also, when they are spamming block, a power attack can break through.

The critical damage perk increases power attack damage as well.

use two daggers hold both down at the same time then come back and tell me is it worth it