My 7" model works similarly to the 5.75" coil, I assume, picks up the smallest items, fantastic on chains and great on everything else.
My sensitivity is turned up to max internally, (the only way I would use one), and is super hot so I get pretty deep.
I had a chance to mess around with a new 8" coil model last summer, right out of the box is seems powerful and deep with no mods, still does the double beep thing next to big metal so you can find targets very few others would even know were there.
I did some air testing while I had that 8" model.
Keep in mind this is indoors, I turned mine off but I have about 10 other wifi sources around me, and if I would have tested these targets in the middle of the coil instead of near the rim I think the depth would have been a bit better.
[video]https://www.youtube.com/watch?v=GMIl2qCjsE8[/video]
https://www.youtube.com/watch?v=TYmVdgcybUQ
https://www.youtube.com/watch?v=3Vh2gz_7jeg
If you hunt mostly real trashy areas get the small coil version.
Large open spaces the big coil is better.
If you do both I would probably get the 8" because it seems to work just about as well as the smaller coil with superb target separation...or just about the same as my 7" coil model, anyway.