Just a quick note - while most "battery tube" mods perform the same, the P+ series uses rhodium plated contacts which really do make a difference in my opinion. I'm not aware of any other mods out there that can say the same.
Not to say the Don is a bad choice at all - they're both very good, quality mods. If you're looking at ONLY using 18650 batteries, and don't mind spending a little extra cash, I say P+ all the way. If you want some flexibility, and don't mind sacrificing quality and want something a little cheaper, than go for the Don. You can't really lose either way, in my personal opinion.