Trigger capacative button
I believe it is worth trying to connect directly to the spring: you may get more consistent triggering (I would expect it, but no guarantee).

I'd use a small ceramic capacitor instead of a direct connection though, to protect the capacitive sensor from DC and 50 Hz AC currents which may appear in your circuit. It should be anywhere in 1-100 nF range, one terminal connected to the spring, the other one to your relay. I actually expect such capacitor to be present on the PCB in series with the spring, but extra protection can't hurt.