I started with the cigarette-sized batteries. The Smokeless Image Volts. I had a couple of the manuals and a couple of the auto's.
Some people like the auto's for a more "cigarette-like" experience for the transition of smoking to
vaping. Some others also like that they can hold the battery in their mouth like an analog to allow them to be hands-free to to other things while they vape.
I prefered the manual button. It allows you to adjust the vapor easier in certain situations. Sometimes it's necessary to do a "primer puff" (puff without power on) of a cartridge or carto if the juice is too thick or otherwise just not wicking well; can't do that with an auto. Some people like to pre-heat the juice before taking a draw; can't do that with an auto.
Most auto batteries are not "sealed", which means that liquid that leaks into the battery from the juice delivery device (it seems this happens to everyone) will short it out and kill the battery. Even auto batteries that make the claim that they are sealed (Smokeless Image Volt autos), still have an air hole present (it's what allows the chip in the battery know you are taking a drag) which leads to the business end of the battery and can allow liquid to get in there and kill the battery. Manual batteries are completely sealed off and do not have this problem.