Hi,
I may be wrong here, but I don't believe you can swap the coils on that model. I looked at the manual and they don't mention it or show any other coils as an option. On other BH models that have the spider coil standard, you can use either the 4", 8", or 10". In answer to your question, the closed coil that your detector has will work fine. The spider coil is easier to pin point with as you can see exactly which blade of grass is in the center of the loop. They do tend to snag on brush and deeper grass more than the closed face coil, however. The 8" spider coil will probably have a tad bit more depth capability that the 7 1/4" coil due to it's slight increase in size. Just my thoughts, hope this helps.