I finally decided to move my wife from SO to the $15 plan at Wind, which I got for $7.50 per month for 6 months (no contract, this offer ends on the 31st). I was hesitant to try them at launch last year since I didn't want to experience their startup issues, but now the rep in the Wind store seemed to have her act together. Here is my rationale:

My wife has been using about $15 per month, some months higher, so she is in the 75-100 minute range with the $100 vouchers. We don't want to get voicemail on our cell phones so the lack of VM in the $15 Wind plan was not an issue for us.

With Wind, she will get 100 mins for $15 ($7.50 initially) or about $.15/min. Extra minutes are $.10 in the Wind zone, $.25 out of the Wind zone. Out of the zone in the places we travel to seems to be LD for SO anyway, so I am comparing the $.25 to the SO rate of $.20 pus LD.

Another plus is that we can roam on T-Mobile in the US at $.25/min. I have been thinking of buying a second T-Mobile number so that we can find each other in the mall for our US travels, but you have to spend $100 with them to get to their best rate of $.10/minute.

Not having to spend for another US phone offsets the cost of buying a new Wind AWS handset. But I think that 5-band phone from Wind is going to come in handy on our foreign travels, after I get it unlocked.