Howard is a remarkably consistent goalie. I'd say he and Miller are on the same plane (top 5 goalies), so it's a lateral move. Might as well stick with the guy we got, although I like Miller and lot and wish he could have some degree of success somewhere.
But if we're getting a high-end defenseman in return for Howard, and then trading maybe a decent roster player, a high-ish pick and a solid prospect for Miller, that might be more than lateral.
My knee-jerk reaction is, of course, "No deal!" - but I'd want to know exactly what we'd be getting in return for Howard and exactly what we'd be giving up for Miller. Because, if I understand correctly, we're not talking about just a straight-up goalie swap.