Kulak is more mobile and more offensive, but also less reliable. Kovacevic is more of a steady guy who does his job quietly, though he's joining the attack more these days (and I'm not sure it's his game).
See him as a complementary D on the bottom 4, who can play on the PK and is never a liability.
For a 2nd and a 7th, I'd be listening, frankly, even though Montreal has no reason to move him - he fills his role just fine at a very low salary, and we can't ice a team of 21 year old rookies and 2nd year players.