Hi taternut,
Congratulations on your first piece of gold with the MD. Awesome.
I am nearly as new to metal detecting as you. I started detecting for gold with the GB Pro about 5 months ago. For the first 4 months I used the 5" round coil and found some nice pieces of gold. Three weeks ago, I bought the 10" elliptical coil and have used it only about a dozen times so my observations might change with more time and experience. However, in these past three weeks I have brought both coils with me when I detect and have been experimenting extensively. I have tested both coils in various conditions, geology, ground mineralization, in trashy areas, among hot rocks, etc. This is some of what I have learned so far...
--Since it is a larger coil, the ground coverage of the 10" coil is far better than the 5" round. Because of the geology in our area, you don't need to stick to waterways and locations that show evidence of mining to find gold. I have found early mining records indicating concentrations of gold in random surface patches all over this area. Although a knowledge of local geology has helped me greatly, because most of these patches are produced randomly, I spend a lot of time swinging blind. What I mean by this is that I detect areas away from waterways, that have never been mined, and that don't necessarily exhibit a geological setting that is especially conducive to the occurrence of gold. Having a larger coil has allowed me to more thoroughly cover an area, in less time, and while using less battery life.
--The tip of the elliptical coil is tapered to a point enabling it to reach into tighter spots than the round coil. The day after I bought the 10" I found a piece of gold that was situated amid a cluster of manzanita trunks. This piece was large enough and shallow enough to have been found with the 5", except for the simple fact that the 5" could not fit into such a tight spot. Although the round coil is small, when I'm detecting in a thicket or between cracks in a rocky outcrop, an extra inch is significant.
--The 10" coil is as sensitive as the 5" coil to small gold. When I bought the 10" coil, the guy at the shop told me that it was less sensitive to small (grain-size) gold than the 5", but more sensitive to larger gold. At first I didn't like the trade off, but after much experimenting I am certain that the 10" is finding small gold at the same depths as the 5" coil.
--The 10" coil is more sensitive than the 5" coil to larger gold that is deeper. At one of the areas I like to detect, there is an "island" of shallow bedrock surrounded by deeper sediments. I had completely detected the shallow areas many times with the 5" coil resulting in 16 pieces of gold. However, as soon as I got to the deeper areas... nothing, not even lead. I returned to this area recently with the 10" and detected the surrounding deeper areas. Along with many more lead signals that I had missed with the round coil, about five inches down I uncovered the largest piece of gold I have ever found. Afterward, out of curiosity I switched to the 5" round to see if it could've detected this piece. It did, but only at around 4 inches. Again, an extra inch is significant.
--The 10" coil is more sensitive to being bumped than the 5" coil. This is really the only problem I have with the 10". It seems that just about every bump gives out a gold tone and gold reading. Many times I find myself focusing more on keeping the coil from bumping into things and less on the display.
Both coils are excellent and very sensitive and I am pleased with both. Before you consider the spending $180 for a new coil, you must realize that depth-wise only 1 of my 22 pieces of gold found with the GB Pro was out of range for the 5" coil. I would suggest that you keep using the 5" round until you determine, if you ever do, that it is limiting you. There is more gold-sprinkled shallow bedrock out there than both of us will ever find, or detect.
