It really depends on what you do
For example: For Doubles, Steel 108's.
For a Single tank, I use LP 95 Steel.
For the OW students, AL 80
I use to own 100's but they were hard to fill to working
pressure 3442.. So I went to LP 95's with a working
pressure of 2400, which is easy to fill to that pressure if
the shop is filling AL80's to 3000... Any pressure over 2400
is a bonus...
Keep track of your SAC equivalent for each dive to help you
determine the amount of gas for the time and depth you like
to dive to determine the amount of gas to carry, hense the
cylinder you require to hold that volume with reserve to spare.
http://www.dive-logs.com/infosac.jsp
http://D*D.com/article/make_the_most_of_every_tank
http://home.flash.net/~table/gasses/sac.htm
Lars