The 10 inch is a good size, you'd only need to get bigger if you had properties with very deep sills. Effectively you only need to buy one even if you have 2 poles, because the goosenecks are quick release, however I would recommend getting one with an extreme brush and one with a radias sill.
The best thing about these goosenecks is the weight, you can't overemphasis how important lightness is at the topof the pole.
Simon.