Amps (in our case milliamp*hours) = "Juice", how much energy there is in the pack for "work" (ie. turning the motor/gears/pulling the spring etc.).
Voltage = "Push", how much power the batteries (in series) can push through the wires, more voltage = more trigger response and generally more ROF. Keep note that you may need a higher voltage battery to turn tougher springs (as well as a high torque motor/gearset)
Also note that generally 1 mAh = 1 BB fired (or one turning of the gears/motor) but it's not exact depending on environmental conditions (ie. humidity, temp, weather, etc.)
These are INDEPENDENT OF EACH OTHER high voltage does not mean high amperage and vice versa. What I mean by that is that you can have a 12v battery that only packs 500 mAh or a 1.5V battery that packs 2000 mAh (these are extreme examples).
As for "cheapbatterypacks.com", get your batteries from there and there only if purchasing online or from a local RC hobby shop depending on how you feel (ie. support Canadian businesses).
Also keep note that differing gauges of wires have differing resistances. It's suggested that you stick with 18 or 20 AWG wire though because of possible fitment issues when inserting into the battery compartment. A thicker wire will theoretically have less resistance than a thinner wire (of the same material differing materials also provide differing amounts of resistance) however the real "bottleneck" is actually the connector and Deans or Traxxas connectors are what's recommended (use deans since it's a lot more common in Airsoft). You will see a definite increase in responsiveness and ROF if switching from something like a mini tamiya connector to a Deans connector.
PS: Squeenix mAh stands for milliamp*hours ("mA*h") how much charge can go through a piece of wire in an hour while an Ampere is charge per unit time (you can have Ampere*hours as well).
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE