OK, first off, as far as the brands you mention, it depends where you are. Buy locally, it'll be a lot easier to use the guarantee. If you are in the US, get Njoy or Janty. If you are in the EU buy Janty or Sedansa.
Second, as to the brand vs generic debate, you can now literally pay half as much for a generic as for a brand, but if it dies after 3 months, or sometimes 3 weeks, you're totally out the money and have to buy a new atomizer, batteries, whatever. Some just budget $10 a month or whatever for replacements, but that adds up. In the long run a brand name with guaranteed replacement parts for a year is a better value.
But that only works IF you get it locally, and can make use of that guarantee without long waits or import taxes. I understand that's a problem if youre in the UK, so if you're from there, I'd agree with the generic boosters, that's your best option there, and I would suggest Smokester.