I would use a connector rather than soldering the ignition wire, and proper crimping is more reliable in a vibrational environment than soldering for cabling. Soldering is for circuit boards. Crimping is for cables.
The "ignition wire" supplies power to the logic circuits in the controller, and any accessories. The other "main power wire" supplies power to the FETs that power the motor, and the large capacitors that they have. So connecting this wire causes the capacitors to charge suddenly and creates the big spark that damages switches and connectors.
The "ignition wire" powers a low power circuit, and doesn't have the big capacitor bank charging current surge and spark issues. It is the proper way to turn the system on and off, whether by a switch, keyswitch, or small connector. I think all bikes should have a handlebar mounted "kill switch" on this ignition wire, like a safety on a firearm it allows the bike to be kept "off" until it is necessary and safe to energize it. Don't ask about those tire marks on the wall or ceiling and why this is a good idea.
The "main power wire" should be disconnected when the bike is stored for longer periods, and should have a "precharge circuit" to slow down the charging of the capacitor bank, and preserve the contact surfaces of the connectors and switches.
Some capacitor banks have bleeder resistors (common in low cost controllers). These bleeder resistors will discharge the battery and possibly damage it over weeks of storage. Better controllers don't have this wasteful bleeder and the "main power" can be left on for long periods of time without discharging the battery pack.