Joule is a high mobility hero. Her base health is higher compared to Catherine or Petal. With her Rocket Leap it can do wonders and change the tide of of a battle if you land the stun. Look for opportunities to use your Thunder Strike on multiple enemies. I will explain everything later.
Why Weapon Power?
I Thought of weapon power as Thunder Strike damage is based on 50% weapon power and 20% crystal power. And if you can spam it... Why not go weapon power? If you go crystal power you can deal a lot damage with your Rocket Leap, but if you miss it, you either deal 1/3 the damage or simply misses completely. Your ultimate is based on 400% crystal power, so you can probably use it for one shot kill a squishy target. However, it is, after all, an ultimate and it have high Cooldown. And that's when Thunder Strike, the spamming ability, comes in handy.
Why Crystal Power?
Crystal power will be tons of fun. Many people do this because the ultimate have 400% Crystal poeer scaling, making it very tempting? Wait, that means you can one shot turrets?! Nope. It deals half damage to them. So it will be wiser to use on enemy heroes. Also, your Rocket Leap will deal tons of damage... If you land it. If not, the damage will be 1/3, so I suggest going weapon power for beginners.