I know enough to answer these. The mah rating is a measure of the capacity of the battery so 900mah battery holda 50% more than the 600 mah battery. Not all batteries discharge at the same rate so you can't exactly compare and say the 900mah battery would last 50% longer, it'd be more of a range say 35%65% longer.
You can calculate C, the discharge rate, needed for any device based on basic electrical properties like resistance (ohms) the voltage involved and so on. When you do the calculations for the LR (low resistance) atomizers you get a drain rate that's noticeably higher than the drain rate of a regular battery. In the 14500 size case that's the 900mah one. Even a normal resistance atomizer results in discharge rates near or just above the rate of a normal battery. But, and this is the issue folks debate, you aren't running the battery at that rate all the time. You only discharge the battery when you vape. The rest of the time there is no discharge of current from the battery at all. They argue that since its just a relatively momentary discharge at those high rates it doesn't matter nearly as much as if the battery would be used like that continuously. So some people feel there is no need for high drain batteries, others feel a high drain should be used for all atomizers and others say just for LR atomizers.
Add in that the maker of the Wetbox doesn't recommend LR cartos and won't warrant the wetbox if its used with them and you have less need for a IMR battery. Unless you're in the IMR should be used in all cases camp