Underarm zips are a nice addition but you'll pay for them. The breathable laminated fabrics like Gore-tex are designed to allow the moisture generated by your body heat to escape through the fabric. The theory is that you won't get wet from the sweat you work up while walking. The jacket is not supposed to let rain in, just the body vapors out. You will pay a price for this luxury and whether you decide to go for it or not is up to you.
Marine stores carry great rain protection.
Keep in mind the weight of the jacket because if it's too heavy or bulky you're less likely to want to carry it with you all the time and, well, we've already gone over that.