I would personally go with DL for the simple reason that DCA typically closes earlier and I'd want as much time in the parks as I could get! But I, too, really enjoy PTN and find it's so much easier to get a good spot without camping out when you're solo. You may be planning to see it with your family, but I think it's worth enjoying again.
I'm not sure what day you're going or if there will be a second PTN parade, but if there are two, I'd probably enjoy a dinner (or snack) at the French Market/etc area while soaking up the atmosphere of RoA in the evening without the congestion of F!, maybe go on a couple of rides (Pirates, HM) then while the first showing of PTN is going, make my way through Fantasyland after the parade (which is so pretty at night!) to the Matterhorn. I know you say you're not a fan of roller coasters, but riding the Matterhorn at night makes it feel like a whole new experience, plus you could go in the single-rider line and perhaps not wait as long, and sometimes if you time right, you'll ride during the fireworks which makes it seem extra special. Then about twenty minutes before the second PTN, I'd grab a churro (or other fave snack) and find a spot along IASW to wait for the parade. By the time it ends, the park may be close to closing so then I'd hang out a little longer, maybe hopping on a few Fantasyland rides, before leisurely strolling out, taking my time to take nighttime photos, maybe do a bit of shopping.
If it's a weekday night when DL closes at 9pm or if there's only one PTN, I'd probably do a similar itinerary just modified to allow for the time to go catch the parade at IASW (maybe getting there more like a half-hour earlier than 15-20 minutes like you can do with the second parade) with, again, a favorite snack. I might skip the evening Fantasyland rides and instead scurry over to NOS in the last half hour to get on HM and Pirates (because they're my favorites and the lines are usually really short near closing).
I would also check the showtimes for that day to see if there would be any shows (such as Mickey and the Magical Map or the ones at the Royal Theatre) that would have a late enough performance and then add those in, just because I love those performances so much I'd be happy to skip the chance to ride something just to watch them again.
Overall, I think DL is a great place to soak up the atmosphere in the evening, provided you're avoiding the Main Street congestion leading up to/during/right after the parade and fireworks.
(On the other hand, DCA at night is fun, too, especially Radiator Springs! Plus it's fun to grab an [alcoholic] beverage and people watch. Either way, you can't really go wrong -- it's an extra night in Disney!)
The original park baby! 
