Depends on what you are doing.
The Explorer excels in two things - pulling coins out of iron and getting the deep stuff.
Your 8" coil is good for working trash and getting in where the larger coils have trouble. Fewer targets under the coil equates into less confusion.
The 10.5" goes deeper (not by much, though) AND it covers more ground. I would use it on ball fields and other sparse trash areas where you want to cover the most ground.
My experience with depth is thus... I have an 7.25" Minelab coil. Using a penny as the benchmark, I have dug them down to about 7 inches with the 7.25" and down to 9-9.5" with the stock coil.
The larger coils excel at finding coins on edge. I haven't found any on edge with my 7.25" coil. They can also punch through mineralization and let you pluck some coins out of cinder and the like. The power is there with the larger coils.
You also do not get a 1:1 return for your coil diameter when working packed earth. The deeper you go, the lesser your returns. The WOT might go an inch, inch and half deeper on coin sized targets than the 10.5 inch coil. But iffy, mid-range targets become solid with the larger coils. This doesn't hold true on the beaches, though - bigger is better in loose sand.
There are some truths that aren't so obvious. I searched with a WOT exclusively for weeks. When I go down to the 8 inch coil, it is like using a little scalpel. Working with a bigger coil will absolutely improve your skills with the smaller, more managable coils.
Another truth is that 90% of the coins I dig are no deeper than 5 inches. Not that the XS can't get down deeper with ease, but that is just where the coins are hanging out around here. The majority of my oldies have come from high trash / mineralization conditions in areas that have been heavily hunted.
It is my opinion that the mid-range power of the larger coils is more important than the depth factor.
The smaller coils excel at handling dense trash - and they are also easier to use because you have few potential targets under the coil. They are also more stable to operate, allowing you to push the sensativity up pretty high which means that you can maximize your depth with ease.
But you knew all that stuff <IMG SRC="/forums/images/smile.gif" BORDER=0 ALT="
">. You should use the bigger coil - go find a baseball field and let 'er rip. Next time you use the 8", you will marvel at the precision.
Tim