Good information here already. For a 1 to 2 ohm vaper at relatively low wattage the Pico 75W will generally get longer life from a battery with high MAH. If you plan to push the Pico to higher wattage (50 to 75 watts), then you need to look for a combination of higher MAH and higher amperage rating.
A good parallel comparison is horsepower (MAH) vs torque (AMPS) in a vehicle. Horsepower gives you pep and quick acceleration and torque gives you pulling power for heavy loads. If you have both, then you can accelerate and also pull a trailer when needed.
I vape at 10W most of the time. At 10W I'm pulling around 4A from my Samsung 30Q or my Sony VTC6. My 3000MAH batteries give me long vape times on a charge.
As Zoidman pointed out, the batteries in your variable mod supply power to the electronics. The electronics supply power in variable mode to the atomizer coil. The mod will choose how it uses the power from the battery to create variable output as needed. You just have to pick a battery that can supply power for the max load the electronics can handle. The battery with the highest MAH rating that also has the current rating needed to run the mod is what you need.
Sony VTC6, Samsung 30Q, and LG HG2 are some of the most popular cells here on ECF.