Which one and why? I don't currently plan on hooking up 2 subs, but that could change in the future.
The version of Audyssey used on the 515 (2EQ) cannot EQ a sub while the MultEQ version on the 1613 can as well as having 2x the number of speaker EQ control points as 2EQ.
Get the Denon 1613. But for like $35 more you can get the Onkyo 616. In that case get the onkyo 616. He 616 still has Audessey 2Eq but adds 5eq band, plus 4eq bands dedicated for sub, a 3 stage inverter amps, 20 more watts per channel and thx certified.