Played DDS1 and... failed to finish that game. Dungeons are super long and didn`t find the battles to be all that interesting... Didn`t find the story or characters all that likeable to bear through it x_x; so bleh.
To be honest, my problem with the story is the endings, but I haven't yet seen one of them yet and I heard that one is the best and definitive one. The story itself, atmosphere and characters are all great however, especially the atmosphere. The battles are really good.
And I wouldn't worry about Nocturne aging well, the only thing out-dated about it is the random battles, which I found surprisingly tolerable considering my hatred towards the mechanic. Ever played Digital Devil Saga? It's a bit like it.
Not necessarily a matter of difficulty or anything... as you mentioned, if the story is terrible (hard to follow) then the battle better be *real good* or else I`ll end up quitting =( My friend said that the older SMTs are great but they `don`t age well`... so that`s why I`m hesitant.
Nocturne's great. It's different from other JRPGs in that stats don't win your battles but your strategy and team will, making each boss play more like a puzzle. It's a bit like Persona 3 and 4 in that, just puts more emphasis in it. It's great fun managing your team and the battle system is better than P3&4, but the story falls flat. I recommend it, and since you played Persona 3 (and 4 I assume) you won't really find it as hard as some make it out to be. (Hell I find it easy so far.)
I actually don't like Persona 3 that much because Tartarus is awful and I prefer Akihiko if only because he boxes. I haven't beaten the game yet since I'm playing SMT Nocturne instead, so maybe it'll change.