I love my 24mm II, and it works pretty good for churches; however, if I was going to get a prime specifically for architecture, I would get one of the TS-E lenses, like the 17mm. Another possibility if you haven't considered it already is to get a panoramic tripod head like the Nodal Ninja and use stitching to build wide angles from multiple shots. That lets you use a very high quality prime lens (even telephotos, like a 100mm macro), but the downside is motion in the frame and extra post processing work.