Depending on what type of detecting you do, I would recommend a smaller coil as your first addition. You will be surprised how many more good signals you get out of a trashy area with a smaller coil. My favorite is the Sun Ray Stealth-8 (8 inch coil). It is very lightweight, punches down deep and very durable. The Minelab 8" is a tiny bit smaller and quite a bit heavier than the SR 8". I have no experience with the SEF coils everyone is using, but the 6"x8" might be a the ticket - for me the light weight of the Sun Ray is a major factor.
I started by saying "depending on the type of detecting you do" - by that I mean, if you hunt coins in trashy areas - go with a smaller coil addition. If you only hunt on the beach in fairly trash free ground, go with a larger coil for more coverage and depth. ML Pro 11" coil is good but not a lot of depth difference from the standard coil. It is lighter and better balanced.
I have had many coils, the Stealth-8 is a keeper. Had mine for 8-9 years.
Hope this helps. HH - BF