Pretty tuff questions actually, because leather weight and it's "tan" is directly related too it's flexability and use.
For bag making, you kinda need something that's going to hold it's shape when stitched into the shape you want and
not act like a plastic bag full of water when gear is tossed in the bag.
Thickness chart;
http://www.tandyleatherfactory.com/en-usd/home/infoandservices/leatherguide/leatherguide.aspx
And Tanning techniques;
Veg tan generally leaves Leather less flexable,
3-4oz Veg Tan is flexable enough for bags and will retain shape, but you get into 5-6-7oz and it's more like a belt, saddle or tooling grade leather.
Chrome Tan leaves leather more flexable in a heavier weight and a 6-7oz will make a nice bag and used by many market makers. I have an October Country that's .090 thick and I've used some thats .110 and still nice.
Both Veg Tan and Chrome Tan can be "oiled". Not a big deal when choosing one or the other because you can treat your pouch with any of several products to "water proof" the finished bag.
http://www.ldsleather.com/leather_tutorial.html
Braintan anything is usually soft,Buckskin can be some pretty thin stuff, I use it for small "medicine/tobacco" pouches and as a covering for leather or rawhide sheath's but it doesn't have enough body for a larger bag by itself.
Brain tan Elk hide makes a good bag when it get's up around 6-7oz or more, Buffalo is even thicker.
Good luck, Other's will chime in too