What is the difference between the thin bosals and the thick

  I mean is one more comfortable and one harsher. Which would you use on a horse that is new to a bosal. I'm confused because I've heard them both called training bosals.
Just like a thicker bit compared to a thinner bit. A thicker one will spread the pressure over a larger surface, so it's a tad bit milder. A thinner one will have a slightly sharper pressure.

It's also a good idea to only buy bosals that have rawhide cores. A rawhide core will bend and should be very flexible, so it will form to the horse's face much better. Some bosals are made with metal cores, which means they can't be shaped to the horse's face and they're a bit heavier.

Look for a bosal with a rawhide core and a higher plait. It's hard to explain how to figure out the plait, so google it. It's something like how many strands of rawhide cross under one little rectangle, multiply that number by 3 (or 4 , don't remember), and you have the plaint number. The higher the plait, the higher quality the bosal.
Everyone has an opinion on the size & weight of a Bosal and their use. Look into the trainers that use them ... follow the advice of the trainer that you feel most confident with.