I rarely take the 5x8 off. If you are searching primarily fields the only thing you are going to gain with the 5x8 is balance of the machine and a lighter swing. The 5x8 and 8x11 both reach deep. I prefer the 5x8 because at least once a day I find myself in a junky area where the 5x8 shines with discriminating the close together junk/good signals.
I do use the 8x11 when shooting fields though (as long as I won't be out too long due to the weight) just because of the extra coverage.
Smaller Coils are much better for Target seperation....the Larger Coils are good for large "Clean" Areas....if you are in a junky place..Small coil...if you are walking around for 5 minutes every signal..then the Large coil is good..
its all about where and what you are hunting. I used to have the ACE 250 small coil...and a year or so after, got the bigger coil