realskiers

No ability to put them in order, but I would consider a group of the Head XRC500 153, Head Cool Thang 156, Fischer RX6 at 150.  I think all of these are probably pretty similar in ability, the RX6 may actually be closer to the XRC800 (but I don't know much about the RX6).  I guess if all are available, then pick the Cool Thang because it is a woman specific design, but I would doubt that your wife would notice much difference in skiing them.

Purely based on the RX6 review, 150 is probably the right size for the RX6.  The reviews for the RX6 list 160 as the tested size, so it is likely that it was designed to be skied a little bit shorter.

10cm difference in length should only be about 4 inches, not well above head to well below the chin (TruLuv - RX6)   I think Gary once told me that the difference in sizes has to do with the style of measuring.  Some manufacturers measure the total length of the ski, others measure the length that touches the snow?