your wife needs the top tube of the M,so I would say the X/M.
I'm 6',wife is 5/6",we have the X/M.The front seems to be a 60cm TT,not 60.5 if that helps.
We bought the X/S first,and the wife was kinda cramped.She likes the M rear better.
Also,I prefer the higher top tube of the X/M beacause it makes it easier to crouch down and stabilize the bike during mounts/dismounts. The top tube was waay low on the X/S for me.
Cannondale needs to resize the front of these frames-a 4 cm top tube jump is dumb=it should have a ~59cm top tube in the X front.
We are about 75 miles south of Pittsburgh PA if you are anywhere close and want to try it.