I bought a WOT 15inch awhile ago thinking it would be great for beach detecting where you need to cover a lot of beach real estate in a day. It is a deep detecting coil BUT it is on the heavy side and I ended up going back to the stock 11inch coil. The 11inch coil is much more easier to swing for a period of time without getting too tired or when your shoulder starts really aching/burning telling you it's time to quit. At least this is true for me.
If you have youth on your side and can manage to swing the 15inch without getting shoulder and back ache etc; go for it....at least on a beach or big wide open pastures/fields. I would not use it for parks and local detecting because it will detect all the trash under that big coil footprint and will probably mask out coins or small rings and small jewelry items. In the end, it's a personal preference decision.