I think I'll be buying "boys" knickers for dd for as long as I can get away with it. They are made better.
When looking for a good pair of jeans for ds when aged 4 I was in gap and found some good ones in the girls area, but didn't at first notice it was a 'girls' area. They were slim (he's narrow) and soft which I thought would be nice to wear. I then had a look in the boys area and found what I thought would fit (most were very baggy) but the material felt tougher and stronger - at first I felt they'd be uncomfortable. I went to and fro and noticed:
On the girls: smaller or no pockets
Slimmer, softer, stretchy but ultimately not as hard wearing, no turn ups.
On the boys: thicker tougher material with less stretch, turn ups for making them last longer, bigger and more pockets.
In the end I bought the boys ones as I think would have done for a dd. They did loosen up and have been a great buy. I know the others would have worn through quickly.