Both candidate pants are pretty good. Ex Officio's Explorer Pants ($56) are made of a lightweight nylon that dries quickly and is pretty wrinkle-resistant. One nice feature is a gusseted inseam, which means an extra piece of fabric is stitched through the crotch area, allowing better freedom of movement. That helps make the Explorer a good adventure pant that also travels well in civilized society. Royal Robbins' Go Everywhere Pants ($65) are very similar, differing mainly in the use of a branded nylon fabric (Supplex) versus the Explorer's more generic fabric. Does that matter? Probably not a bit. I'd say that performance for the two would be very similar, so let fit be your guide.
You might also look at Patagonia's Tropical Flats pants ($65). They're made with a fabric that blends nylon and polyester for a little better performance than nylon alone. In particular, they'll dry faster because polyester absorbs much less water than nylon, which actually is surprisingly hydrophilic for a synthetic.
So, take your girlfriend to the store with you and model some pants for her. Buy whatever she likes.