Can't speak for the ND5 SX because I haven't heard it, but I can state that the SM6 is very good indeed - certainly on a par with the streamer in the Uniti.
As far as usability is concerned, I've used both apps at length and tbh whilst on the iPhone the Naim app probably shades it, on the iPad they function almost identically, and I wouldn't use that as a reason to buy one or the other; I would point out that both need the app to make them usable at all though.
It's quite a big price differential, so I'd want to be absolutely sure that any uplift in audio performance - together with the admittedly superior build of the Naim - is worth it to you.