First, you must choose [you can't have both!] between the following:
  • High velocity and down range power 
  • Long band life
If you are looking for decent power and decent bandlife, pay attention to the +P bandsets
If you are looking for longevity and ease of use, pay attention to the Match bandsets
If you are looking to slay dinosaurs and destroy your target, pay attention to the Nitro bandsets
Remember, just because the bands have a heavy draw DOES NOT mean that it will shoot your preferred ammo faster or harder.  Even though we are dealing with slingshots here, precision and science do have a very real and useful place in your decision making.
Once you determine your general use, the next step is to pick your chosen ammo.  Not all bandsets will shoot all ammo well and each bandset will have a range of ammo it will shoot well depending on the active band length.
Each bandset section, whether Match, +P, or Nitro will provide a range of ammo sizes.  These sizes are for steel ball ammo (a comparison chart can be found here).  All information regarding the draw weight is based on a 30" draw length.
For example:
You wish to shoot 3/8" steel ammo and your draw length is 28"(shorter than the 30" average).  You want to maximize band life at the expense of speed and you really like .040" natural latex.
You have two choices:
Match 5/16"-3/8" single layer
Match 3/8"-7/16" single layer
Given your shorter than average draw length, you will not be stretching out the bands to achieve maximum power.  As such you have two choices:
1- You may purchase the 5/16"-3/8" single layer and shorten the bands in small increments from the fork end until you achieve the speed you are looking for.  You will slightly reduce band life in doing this, but the speed will increase
2- You may purchase the 3/8"-7/16" single layer and shoot them as is.  You may not see as much speed as the previous choice, but the bands will last much longer.
So, as you can see, slingshot bands are all about tradeoffs. Choose whichever combination fits your preference best, and experiment. Over time you'll find your favorite combinations.