Of the two, I can actually see them preferring to get Miller into the fold before Lundkvist. The former is ready physically but needs continued coaching; the latter is pretty close to ready mentally but needs at least another year (or more) to be ready to handle the rigors of NA pro hockey.
Lundkvist simply needs to keep developing his strength and confidence, which can be better done where he won't have monsters hungry (thirsty?)

for a cup of coffee in the NHL constantly mauling him. Miller, on the other hand, could easily slough off most AHL players, and they may well want him somewhere where a) hockey is his sole focus, and b) they can take over his coaching. (Also, there's the *slight* risk he'll go to FA if you let him get too close to the deadline.)
Definitely don't want Lundkvist coming over next year, but I'm indifferent as to whether Miller does or not – up to the Rangers whether they like the coaching he's receiving in Wisconsin.