I don't have a MX331. But I do own a SF U2A. It's a time-proven design, I think the first SF U2 was sold in about 2005 (well at least
this review is from April 2005). You can always get SF flashlights quite cheaper than MSRP but not directly from stores - because SF prohibits discounts. I bought my new U2A at about the same price as on the auction.
I like the simple UI - no complicated menus or memory modes, all my friends understand the basic operation after few seconds. It has insane long runtimes on the lower levels.
The only thing I don't like is that the newer U2A has a bit more concentrated beam, it's good if you want to see something in the distance but not so well if you are shining just in front if your feet. But not such a big issue, you can get a FM64 diffuser (I have one on the way to me from USA).
About the waterproofness - it will survive any sort of rain and I would be pretty confident to use it up to 1 meter water depth. But it's not designed for diving of course (why would you use this for diving anyway
). As long as you keep the o-rings well lubed, no water will enter the flashlight.
Btw. I don't see a clip on the MX331? U2 has a clip...
The lowest level on the MX331 has 10 lumens and 65 hours, SF U2 has 3 lumens and 175 hours and even this lowest setting is actually too much for me (night adapted eyes) for reading.
According to the website,
MX331- "Runs on three CR-123A batteries (included)." Three? Well U2 uses two. I am a bit surprised that it actually takes three, the battery tube doesn't look that long...