Another over-priced unit that under-performs. Read around here, and you will see lots of recomendations. 510
threaded stuff has the greatest variety and largest number of vendors atm. I don't think I have ever seen anyone upset about buying a 510 and there are tons of battery size options to choose from. Also a ton of atty/cartos available out there as well.
It's best to choose a unit that has lots of options available for them. I don't think anyone would recomend a proprietary threaded unit unless they didn't know better.