Welcome to CPF! Both are good options, by reputable companies, and you won't go wrong with either one. I'm a little partial to Fenix (as a company) for reliability and build quality, but I'm sure others feel the same about Nitecore. Both companies are generally well regarded in the CPF community. =)
As for alternatives, you could always look at the RovyVon Aurora or Nitecore Tiki if you wanted a different shape or LED type. The Nitecore Tiki (not Tini - Tiki, with a K) and the Rovyvon Aurora are essentially the same light, but the Aurora is slightly overdriven for a brighter high. I've had their old-school A1 for a while; it's my keychain light, and has performed well under a variety of conditions up here in Alaska. If I were doing it over, I'd probably just go with the Nitecore, for the name and reliability. The size and shape disappears on my keychain.
It sounds like you're only going to use the light for a few minutes to visually inspect the aircraft. If that's the case, you won't need much battery life. But if it's going to be longer, you might also want to look at the battery capacity and runtimes. The Fenix has a 200 mah battery, the Nitecore Tini has a 280 mah battery, and the Aurora has a 330 mah battery.
If you're going to use it for your preflight, you might want to choose a flashlight that has immediate access to both high and low modes. You'll want something really bright for checking the fuselage, and something much lower for when you're inside the cockpit. The Fenix doesn't have a memory mode, which means you'll have to cycle through a few modes to get to high each time you fire it up for preflight.
Also think about how important color rendition is. If you're using it to inspect your fuel, you might prefer a high-CRI flashlight, since this type of LED will help you better distinguish colors. (These are sometimes called "neutral" or "warm" LEDs. The neutral's closer to daylight, and warm's closer to an incandescent flashlight color.) Neither of the two lights you mentioned uses this type of LED; if that's an important factor, you might want to consider the Nitecore Tiki. You probably want to know if your fuel is dyed red, blue, or green. ;-)
Hope this helps!
- FITP
