tsfca, your plan and reasoning sound pretty good to me. I stick with UA and AA because I'm US-based and value their free upgrades (by virtue of my elite status) for both domestic and international travel, but those reasons don't apply to your situation.

In addition to the factors you'd stated, there's also the possibility of UA and/or AA going bankrupt, which would probably impact the value of their miles or possibly even the existence of their programs. And as it is, they're already busy devaluing their miles in various ways.