I think I'd probably go with the SF cell, at least initially. Per FL1, that's the cell they used for all the measurements for their specs on their 18350 lights. If there are Support questions / issues, having that single-source also eliminates a few questions / possible finger-pointing regarding the cell(s) you're using. Also, having the USB charging available is a good option to have (whether you generally use it or not), and no worries with the fitment because it's sold for their light(s). Having a single point of responsibility has its advantages, although it's not free - but also if I trust them enough to use their lights, I'll trust them enough to use their cells. They have an extra incentive to ensure those are hi-quality merch, and a good name / reputation to protect. Unlike some companies, they've been around a long time, and I'm confident they'll be around tomorrow. From a product liability / warranty standpoint (if you consider such things), it also has some advantages.