I use the 6" or 4.5" x 7" Excellerator Coils - both are fantastic - the 4.5" x 7" is better at getting into tight spots than the 6" - both get real good depth ( 7" + Dimes) ,great in heavy iron and they are both very light - so you can swing them all day long. Myself, I like the "closed / no open spokes" type coils because they don't hang up on brush and field stubble.
I've read nothing but the best about the 6" x 8" coil from countless users - many use them as their primary coil - some say they hit as deep as the stock Pro Coil.
Here's a link to field tests on a lot of small coils - a ton of good reading here !
http://www.findmall.com/read.php?19,1055627,1055627#msg-1055627
Good Luck !