Orlov is an all-around defenseman that is still young, on a really solid contract. He has the skill and talent of high-end forwards, with good skating ability, physicality, defensive instincts, and offensive playmaking abilities.
He struggled a little with Nisky this year for a period of time - and made some bad turnovers and defensive reads, but, when he came out of that slump towards the end of the RS and into the playoffs, he was easily our most dangerous defender with the puck on his stick and very positive defensively. Carlson is our best defender, but, Orlov can hit another gear and create separation that Carlson cannot.
Niskanen is five years older, costs a bit more, and has been struggling with a decline and lack of foot speed. I think Washington would regret an Orlov trade because they wouldn't get top value for him and he still has another gear offensively that he would be able to hit with less difficult minutes and more PP time, IMO. I also think that Orlov would be better off with Jensen long-term - a defensive defender with good skating ability.
Orlov is perfect for the modern NHL and Niskanen's foot speed and acceleration - lack thereof - is becoming an apparent problem to me.
I don't think that there would be any reason they'd move Orlov before Niskanen.