First was dragonshire, so I went the dragon crash build. It seemed powerful, but my team was so random it was hard to tell as the enemy team had a four level lead on us. One issue is that at 20 you don't have a great choice of talents with this build. Second game was sky temple and I went the sandblast build - which was surprisingly powerful. Between the quest, the piercing talent, the range and the "stack ability power when you hit" talent the damage ramped up really, really fast.
Sandblast (Q) build is where it's at. I wanted to be all in on the W myself, but after seeing the fact that Q is basically the same damage (without talenting W damage, anyway), can pierce, and the quest is very easy to complete...so much damage. The slight delay causes a few misses, but it's a super strong ability.
One thing I would say - the level 7 talent which halves the cooldown on Dragonblast but decreases its damage by 25% seems really, really good. It feels like it helps you contribute a *lot* in team fights, and also in lane battles.
I've went both this and Dragon's Eye. Not sure Mobius Loop is a definite must get though I did really like the CD reduction a ton. The CD is really helpful to shore up her basically non-existent siege/wave clear. You aren't laning long after that anyway, so I wouldn't pick it for that purpose, it's more to clear waves as you are moving around the map. Dragon's Eye turns it into a smaller radius, slightly less damage Precision Strike that you can use every 14 seconds instead of every 60. On the flip side, if you hit two Mobius Looped W's, you are getting more damage than 1 Dragon's Eye W. /shrug. It's also hard to discount how harrowing having the W dropping multiple times in a team fight is with the short CD Q's shooting everywhere too.
I think they are both useful. Dragon's Eye lets you insta-gib at range as the game goes later. It's super strong, but perhaps Mobius Loop will be more synergistic with the Q build because of the rate of fire with your CD's.
As for the level 20's, for shits and giggles take the Andorhal Anomaly. It makes you pretty survivable, sets up nice wombos, and is generally a pain in the ass if anyone is trying to push on your team and you have time to set up (any map with a pushing objective that you can see coming ahead of time, basically). Pretty fun.
What were you doing at 13? I went range every time. It's stupid how much better/easier/safer it makes you.