Are you planning on buying the 11 new? If so the warranty with PE's quality customer service is almost enough alone to justify the extra money. With the SLS you do get all the extra bells and whistles with the SL barrel kit, 2 triggers and HP and LP valves, while with the 11 you get the adjustable solenoid, different ergonomics, angled reg collar and pops asa. It's really a matter of personal preference and budget, but unfortunately I can't tell you which is neccesarily better as I don't own either of them. I do know that you will be more then happy with either you choose to purchase as they are both high quality markers from Eclipse!
I would go to your local pro shop and ask to see an Ego11 and see how it feels and possibly see if you can shoot one at your field.