I haven't had a chance to try a D-28P yet; however, all else being equal, I would generally prefer the slightly wider nut. Based on the specs listed on the Martin web site all else does appear to be equal, and the string spacing at the bridge is also slightly wider, which would make it a better choice for fingerpicking, in my opinion. The ideal Martin dread for me, though, would be an HD-28 with a modified-V neck profile and the neck width of the D-28P.
I, too, prefer a 1 3/4 modified V. I like the extra space to dig in. I like the V in my hands. My hands are probably average size, but I don't mind 1 7/8 mod V necks. I'd rather adjust big than small.
I never liked the neck on the standard D-28. It always felt clunky to me. But the tone has always been to die for.
I have had the D-28P for about a month now and it is the absolute best playing guitar I have ever owned including Taylors, Guilds and Gibsons.. I never thought any guitar would play as well or even a bit better than my Custom Shop Martin 000 but it does.
I have the Lifespan Medium 13's on it and it plays like 12's. The neck is fast and a breeze all the way up. The tone is pure D-28--in my view this is a real winner for Martin or maybe I just lucked out with one of those special guitars that come out every now and then.
It is really hard to put it down once I start playing.