is it better to be on a channel with many AP's, each with a weak signal, or on a channel with only a few AP's, but a strong signal?
As a general rule, you should only use channel 1, channel 6, or channel 11. Of those, when placing a new access point you should choose the channel with the least noise. This includes not just wifi, but also other interference such as microwaves, wireless mice, baby monitors, and all the rest. Since this interference tends to come and go, you need a prolonged measurement to know for sure which channel is the quietest.
Unfortunatley, no free or low-cost tools I am aware of is able to monitor non-wifi noise, mac or pc, and if you're not looking at non-wifi noise anything you do is at best still just a guess. To handle the non-wifi noise, you're usually spending at least a few hundred dollars, and you usually need specialized hardware. Even if you found a tool for your macbook, the need for prolonged measurements means you'll still want dedicated hardware that you can let sit for a few days while measuring.
As for the strategy for a larger deployment, it depends on your density requirements. You don't want to spend more money than you have to, and that means getting by with as few access points as you can to get the job done. Therefore the first step is to determine how much activity will be on the network in a given space. The more activity spread over more clients, the more access points you need.
After you figure out how many access points you need, then you worry about power level/signal strength. More access points means lower power levels. This reduces the cell size covered by each access point, and the goal is to have only one access point per channel (and no more) for a given cell space. Physical limitations usually mean there will need to be some overlap, but as much as you can reduce this the better off you will be.