Well, I have a OLD teknetics 8500 with a box coil (solid bottom) and in tall grass the coil can be laid down on the grass and it will lightly press the grass down and glide over it, with the other detectors spider coils when you try to glide over the grass it will pop up through the webbing, especially pin-pointing and then you have to raise the coil up to get it to move again.
Now, on my 8" 1266 spider coil I found a Tesoro 8" brown donut cover (hole in the middle) that fit the spider coil and it helps the coil to glide better than the more open spider coil.
My Whites 5900 has a donut coil on it and its better (glides) than the spider coil. Now if the grass is a little damp then it will lay down better with either coil making the spider do okay.
When I jumped from years of the solid coil to the spider coil I had a lot of getting use to the spider coil! I had gotten used to scrubbing the ground pin-pointing with the old style coil and when I tried to do that with the spider coil all this grass would start sticking up through the framing of the coil making it hard to move (I had to keep lifting the coil up).
Now the donut coil isn't as bad because the small hole in the center and the large flat area around it works together in that if the grass is pretty tall the flat area will hold a great deal of the grass down, yea, some can stick up through the center but it's nothing like it being able to come up through the hole coil! the spider coil is WAY more open than closed.
So, for the type of hunting I do I would say the donut coil is the best of both worlds.
Now, there is a small problem with the donut coil cover on the spider coil, stuff can collect in the cover much like a pan, but in grass I can pretty much hunt a day and not get enough stuff in it to be a problem and the cover is very easy to get off and clean because of how open it is from the top, for the most part if the stuff that collect in the cover is dry then it just dumps out and if I was going to use it in water I would remove the cover.
Also, if he is getting more depth with the solid bottom coil and that's even more of a plus!
See, the pictures below.
Mark