Nic level is a personal choice and varies widely from one person to the next. We'll need more info in order to answer your question. Namely, what device are you currently using and what's your current nic level?
With regard to the iStick 50W, you should be aware that quite a few people here and on Reddit have complained that the iStick will suddenly start auto-firing. In some of those instances, the device either caught fire or exploded. If you have your heart set on an iStick 50W, I strongly suggest you remove your atomizer whenever you're either charging the device or leaving it unattended for any length of time. As an alternative, you might want to look at the MVP 3 Pro, the Kanger Subbox (which comes bundled with a Subtank Mini), or even an iStick 30W.