I would go with something that drys quickly honestly. If you already have Gore-Tex pants those will do fine. I have a nice pair of ice climbing pants from Mountain Hardwear that are waterproof- breathable and not heavy and have some movement in them so they have lasted quite a number of years. However especially in summer I would rather have something that dries quickly because I will stay cooler and more comfortable. I can keep all the stuff that is mission critical inside a waterproof bag inside panniers and just have shorts on and then when I get to camp or hotel or something I can change to something else (which I would anyways) let the other stuff dry and be more comfortable.

As MikeDeason suggested Waterproof and Breathable don't really co-exist so well though I would take a slightly lesser tone as I have used the stuff for years and while it isn't perfect the right stuff does help a little on the breathability front but mechanical ventilation is really what does the trick.