If you and she do go for the 7.5, it's quite simple to put on a cassette that would be more suited to the terrain and riding conditions you enjoy. Both the 7.5 and the 7.6 lend themselves to upgrading as high as you would want - I just finished customizing what was once a 7.5. But not anymore!
The aluminum used for the frames - Trek calls it Black Aluminum - was shaped using cold-forging methods with water. This supposedly makes for a stronger frame that hasn't been subjected to much stress in it's life. But it sure is lively and responsive. If you need any advice or help if you two do go for a Trek FX - feel free to pm me.