According to this awesome tutorial by @rickkas7, Choosing an Electron sleep mode :
"it takes 334 seconds (about 5 1/2 minutes) of SLEEP_MODE_SOFTPOWEROFF to offset the energy used to reconnect to cellular.
Of course if you need to wake up quickly, you may want to use SLEEP_NETWORK_STANDBY for even longer sleep periods, just to save the 24 to 60 seconds to reconnect to cellular.
The only situation where you’d likely use stop mode sleep (pin + time) without using SLEEP_NETWORK_STANDBY is if you need to wake up to take a measurement frequently, but only upload those measurements infrequently (more than every 15 minutes or so)."
Which brings me to the conclusion that SLEEP_NETWORK_STANDBY would be better suited for my application (if I can figure out how to implement it 
)