Both classes are solid, though I highly recommend making a trial character of each so you can see how they work and which you would prefer. Additionally once you complete the season on one character, once per season you can time piece a character to create a new seasonal character meaning if you feel like you've made the wrong decision, you will have the ability to swap.
#Ninja or Hashashin for Solo PVE
stable star
drifting locust
Do both have decent survivability to just contine PVE or would there be downtime between packs?
stable star
Ideally you shouldn't be getting hit by mobs. If that's something you're worried about I would definitely look into getting at least the first tier of the infinite HP pot