Agree with Northwest, for several reasons.

First, if you fly a route on which Midwest has a nonstop, you can take that and credit the miles to your NW account.

Next, although Milwaukee isn't a hub for NW, it does have flights to a few destinations other than just the hubs of MSP, DTW and MEM...more than you'd typically find for a non-hub city.

Next, when you need to connect, you'd usually go via Detroit or Minneapolis...both far more attractive connection places than O'Hare.

Finally, you said you'd be going to Asia, where NW has a strong presence. From MKE, it's a short hop to Detroit or Minneapolis, from which you can catch a nonstop to Tokyo or Nagoya.