I’m a bit of a newbie and I dont have the same machine as you but I just did some reading up on this subject. Basically the larger coils get you more depth and the smaller coils are good in trashy areas and are better at pin pointing. I just got my new SEF 8x6 butterfly yesterday but haven’t got a chance to try it out yet.
The Garrett 5x8 coil is great with the AT pro, i had a hard time in trashy areas with the larger coil that came with it. The 5x8 is now my default coil unless i'm hunting in an open field where there is not a lot of targets.