I base my decision on which final destination I will frequent the most. In my case, I am a Star Alliance member because I frequently fly to Asia and have found that Star Alliance provides me with the most options for trans-pacific and inter-Asia flights. There is UA, SQ, TG, etc. I don't have a single mile on any One World Alliance, even though AA flies to Asia and CX flies inter-Asia, simply because I felt their options were more limited flight-wise.

Another huge reason I shose Star is because of my hometown - ORD. I felt I could get to more places faster by flying UA rather than AA which would probably connect me to DFW some of the time once my first few flight options have been exhausted.

Both alliance are good... but take a look at your needs first and go from there. I am perfectly happy with my *A choice.