I like to use pre-tinned solid bus wire with a jacket on it. I don't like things to wiggle so when I'm content I'll probably put potting compound over the wires so there is zero wiggling.
As far as gauge, it has to do with the internal resistance of the wire, how long of a run you are doing, the voltage and the current. In the end you have to consider how much power the cable is consuming and being asked to dissipate and what voltage drop is acceptable to you. 28 is right on the edge, and for 5v mods and 510 attys that eat 2.5a, personally I'd go for at least 26 if not 24.
Here's a decent calculator
American Wire Gauge table and AWG Electrical Current Load Limits with skin depth frequencies when it says round trip this is from the + side of the battery, through the switch, atty, and back to the - of the battery.