The Rangers are favored against the Capitals (rightly) just like any other possible matchup they could have gotten. Full-stop.
For a fan, I understand that playing Detroit would be different and novel. Watching the last few games, I thought the Islanders, Red Wings and the Penguins would be the toughest possible matchups with how they are playing. The Red Wings should have lost both games against Montreal, but through will, skill and luck, managed to pull things out - they're got some young players that are continually getting better and don't know their ceiling yet and would be unknown how they would do in a playoff series- so there's reason for optimism in Detroit.
We know what we're getting with Washington, so there shouldn't be any surprises - the coaching staff should be able to put the players in situations where they can execute and have opportunities to be successful.