If you had to choose one, this or Nocturne?
Both are fantastic. DDS is closer to Final Fantasy in terms of structure with the progression, exploration, and larger emphasis on plot and characterization. It's probably the most accessible SMT game for a new player to cut their teeth with, and the closest the series has probably come to a typical jRPG in execution.
Nocturne is fantastic, but emphasizes dungeon crawling a bit more than the plot and narrative. Heavy focus on party construction through demon fusion, recruiting, and customization. It's one large(mostly linear) continuous dungeon essentially, creating the illusion of exploring a world with key story elements slowly unraveling with player choice to dictate the ending.
Can't go wrong with either, but DDS will help get newcomers more accustomed to the SMT components that translate to most all of its games more smoothly than diving directly into Nocturne will. (Though really Nocturne is hardly that overwhelming either).
Any fan of RPG's should really play both DDS1/2 and Nocturne at some point.