Much as my Dalish liked (Divine) Leliana, there's just no way she could so obviously work for the Chantry. The job was done. The promise was fulfilled. It was time to disband, and let everyone go home. Everybody eventually leaves anyway so the Inquisition won't be the same with either option.
I doubt the serve/disband choice will really have much of an effect outside of a few NPC comments. Although it occurs to me that Solas might have a means to "recall" the kill/redeem moment in a Fade vision, I also doubt that choice will have any real impact either. That decision will most likely rest in the hands of the new PC, whoever that turns out to be. If it's well-written, that decision won't be as simple as all that anyway - spare him to "suffer" the world he created, or kill and martyr him for others to continually rally in memory. Only Bioware (perhaps) knows for sure.