I think the big factor for the Titan XD Diesel is the towing capacity. IIRC, it's higher than 1500/150 capacities, but lower than 2500/250s. I'd have to relook to confirm, but that's what I was remember seeing. In terms of the interior, the Titan was definitely better.
However, based on what I think we're going to be using it for (towing my Xterra and two BMW motorcycles, plus a slide in camper in the bed), I just don't think the Titan will cut it. Have to step up to the Silverado 2500 HD Diesel.