This is still a bit of an incomplete hypothetical (though better than many we've seen).

A benefit of accruing MR in addition to UR would of course be to diversify your points collection and increase the number of different carriers/alliances from which you can search for available flights. If you value the flexibility of the increased options over getting the cheapest possible flights, or remaining loyal to a particular airline or alliance, then you should definitely include MR -- Assuming, of course, you plan to accrue enough points in each program to provide a large enough balance to actually do something with.

Your cheapest AUS-SEA option (i think) is to use BA avios to fly the nonstop AS flight (in Y). While BA is a transfer partner with both UR and MR, MR is devaluing its transfer ratio soon, and so you'll be better off with UR there. Adding MR will give you access to Virgin America and Delta for that route.

For the international travels, depending on your specific destinations and preference for coach vs. premium cabins, you might find that Air Canada's aeroplan program (via MR) comes in slightly ahead of United in many instances.

Quite frankly, the biggest consideration for me would be which program offers better earning potential based on my spend patterns. If most of your purchases are travel and dining, you'll get better value from the CSP. If you have a lot of gas and grocery spend, the Amex Everyday Preferred card would beat out the CSP. Non-bonus spend goes on my Everyday Preferred card as well. Utilize the different bonus categories to maximize your total points balance and you'll likely be miles ahead of where you'd be otherwise.