How tall is your wife, and how long is her inseam? (Her measured inseam, not the inseam of her trousers...) I'd think a 17" would fit a tallish woman... 5'9" maybe? But 13" is pretty small. I'm about 5'4" with longish legs (74 cm inseam) and I use a 15.5" or 16".

The difference in standover height might have to do with the height of the bottom bracket. Full suspension XC bikes and especially all-mountain bikes will have a bottom bracket that is higher up off the ground, presumably because you'd need more ground clearance to get over rocks and other obstacles than you would if you were just riding a hardtail on a gravel path, for example. Also, the angle of the top tube might just be different, which can give you a slightly different standover height on two bikes which are supposed to be the same size.

Whichever bike you decide to buy, just make sure to go to the manufacturer's website and check the frame's geometry to make sure that the standover height is at least 1-2" shorter than her inseam. Or better yet, take her to the LBS and have her fitted. Looks are important, but comfort is essential.

~Cari