Another FF program to consider is Aeroplan. Aero miles expire after 7 years from the date miles earned, and it takes 75K miles (vs 65K UA/CO miles) to redeem for a coach ticket to SGN.

You can credit miles from purchased tickets on Star Alliance flights, like UA/CO, ANA, Asiana, and Singapore, to Aeroplan. The Aeroplan online search tool shows award availability on more flights, including Singapore airlines, which are not listed on continental's website. Even if you want to redeem UA/CO miles, the Aeroplan website is a good tool to check award availability.

You can earn Aero miles faster by using an Amex Premier Rewards gold card, which earns 3x on airfare, 2x on gas and groceries, and at lease 3x on shopping via Amex Bonus Mall online. MR points can be transferred 1:1 to Aero miles. Or you can get a Fidelity Amex card, which earns 2 WorldPoints for every dollar spend, and has the option of converting WordPoints 1:1 to Aero miles. You also earn bonus points by shopping online via WorldPoints Mall.

If the goal is to earn free tickets, accumulating Aero miles might be better than UA/CO miles in terms of earning ability on credit card spending, as long as you can accumulate enough miles and use them before they expire.