a Mono Coil sends a round Signal in the ground
like a Funnel

the deeper the Signal the Smaller coverage area
a DD Coil sends a narrow signal
stright down from front to back

like a paint brush
So when you get 8 to 10" down
how much Ground do you wanna Cover ?
The Mono will get a little better Depth
but is it worth it
when you have to be
that tight on your Swings.