The smaller coil allows you to sperate targets in trashy areas. It covers a little less ground and it will allow you to look at smaller peace of the ground and hopefully fewer targets in the smaller space.
I agree with the above tips. I have used an ATPro with a 5X8 coil since March 2012. It is a great combo. I love my Pro but as with any detector you will need to get used to it. It may take some time but well worth the investment. I run my Pro in the custom pro mode with the sensitivity as high as possible, and nothing notched out. The only thing that is turned off is the iron audio. The only other tip I would give you is to swing, listen, dig, swing, listen, dig. Have fun and Good Luck.