Remember - this is a new system!!
I've found the most activity can be found on the 800 MHz system. I've monitored the 700 and missed plenty of traffic on PD, Medcom, and FD talk groups. I chalked it up to being a new system and the possibility that not every radio is associated with every layer yet... or maybe someone is trying to keep public safety on 800, County Functions on 700, and then fill in the gaps with other sites.
We won't know until the system is fully populated.
So for now, try monitoring just the 800 layer. Since it is all simulcast it won't help to know what is the closest tower to your location, but what is the busiest layer on the system.