Hi Ben -

I'd suggest the 24 mm TS-E as your first tilt-shift. One of the reasons is that you can easily add filters, as you pointed out. But for me I just think it is a very good focal length for the kind of landscape shots where tilt-shift really shines. Those shots are the ones where you have a foreground element just inches away from the lens, and the main subject in the background (an example in my neighbourhood would be a wildflower and a mountain). With a 17 mm lens, the foreground becomes weirdly stretched and enlarged compared to the background (you end up with a large flower and tiny mountain). With the 24, it looks a lot more natural. Obviously there are times you want a wider angle than 24 mm, but you already have it with your 14 f/2.8 lens. Or you can stitch images together when shifting, to give you a wider angle without making the background tiny.

Your photos tend to use a lot of creative focus, and I think you'll like what the 24 TS-E can do in that department. You'll no doubt start tilting in all directions to bring out individual elements of a scene. With the 17 TS-E, my guess is that effect will be a bit more difficult to achieve.

For filters, I use the Lee system and it works fine for me. I have a screw-on polarizer and just mount the Lee holder on top of it. I can use one 4x6 filter plus the polarizer without vignetting, or two 4x6 filters without the polarizer. I have a 2-stop hard-edged grad and a couple soft-edged grads, which I use fairly equally. Plus the 10-stopper (which is fantastic). I don't have a reverse grad yet but it's on the wish list. The cool thing about a tilt-shift is you can shift with a grad filter in place, and not have to move the filter. It's excellent for quick panoramas that way.

Using a tilt-shift means being on the tripod 100% of the time, and using live-view for manual focusing. So you'll need a lot more time to take a shot, especially when you add in one or more filters. Note that when you shift the lens, it throws off the camera's light meter (something to do with the angle of the light --- John can explain this much better than me). But the live-view mode is immune to this problem.

Hope that answers your questions!