Great question, I have been pondering the same thing myself. If you check over at findmall Bryce has a bunch of field tests and hunts with these coils all neatly organized. Also includes many other coils. GL and HH!
when i ordered my Etrac last week i talked to a few sponsors getting prices and also wanted a small coil for my Etrac. every sponsor i talked to suggested the X5 coil so that is the one i got.
i just got my Etrac today and wont be able to use it for a bit since we have about 5" of snow that we got yesterday...wouldnt you know.
The 6" DD Excelerator works nicely! I've used it in and around picnic tables and tot lots where there's lots of junk and it easily sniffs out the targets you want.
I brought it to a local park to work around some picnic tables where I know there's tons of junk where the wind has blown stuff off the picnic tables, tin foil and pulltabs, but you can still find the signals you're after.