I am undecided, but if given the change to post 300k to any program I would choose United. I love the Star Alliance web site for checking all flight options. Once you know all of the options it is easy to get to your destination using miles since you can suggest different routing to the agent. I have always been able to call United and book international flights with ease. I also like being able to fly Thai and Singapore as they are both great airlines with better than average service.
I also like United customer service. I am usually routed to the Hawaii center and the people that work there are usually very friendly and helpful. Not something that you find with other airlines.
My second choice would be AA for they seem to have more award seats than others.