Some are better quality, some are used differently, so it's a matter of personal taste.
For example a Hulda Clark Zapper is typically used 7 mins. on and 20 mins. off 3 times in a typical session, and with a T2 or T3 you can wear it as long as you want (best to start slow if you're not used to it though)
These types of Zappers are not programmable, I know there are a couple out there that are.
If you get a T2 or T3 you could also get on the Doctrine explained here: