I have used both companies amps and I have to say both are good. Sundown wins imo in value but JL isnt JL for no reason. I was going to go JL in my ride but I chose sundown instead due to a feeling of better value for my dollar. I also chose them because I like the exterior design. Also I dont think I could get better customer service, but thats also my opinion.
Regardless like stated whichever way you choose I'm sure you will be happy.
Also IMO unless this is gonna be a serious no nonsense SQ build, I doubt you will tell a major difference.