Yeah, full frame is nice for architecture since it works well with the TS-E 24mm. FF+24 is usually good for interiors unless you need to shoot really small rooms like bathrooms - which might need the 17mm.
Using a cropped sensor camera with the 24mm might seem too cramped for interiors but, should be OK for exteriors if you have room to backup. Some places like old narrow European streets may not always have room to back-up when using a 24mm on crop sensor. If you have a cropped sensor you may end up needing/wanting the TS-E 17mm even for large interiors or cramped exterior spaces.
The old Canon TS-E 24mm can be found used around $800-900 maybe new $1000. The new TS-E 24mm II is about $2000 and, the TS-E 17mm is about $2300. Also, you can put filters on the 24mm lenses but, you can't put filters on the 17mm. Very often you'll want to use a polarizer for architecture.
It's also a good idea to go rent TS-Es for a few days to see how you like them before buying.
If you've never used a TS-E be careful with exposure and lens settings. Two main mistakes to avoid.. first, you can't shoot in auto exposure mode with the lens shifted. If the lens is shifted you'll get an incorrect meter reading. You have to shoot manual, make sure the shift is in neutral position, take a meter reading & set exposure then, shift your lens. Or, just use the LCD to judge your exposure setting.
Second, constantly check the Tilt setting to make sure it is always in neutral setting and locked down. It's easy to inadvertently bump it into a tilt even when it's locked and you end up with bad tilt blur when you didn't want it.
Another useful tool for shooting architecture is to get a bubble level that slides onto your hotshot. Traditionally, you always want your vertical lines of the building to be parallel so, the camera should always be perfectly level. You can also use LiveView mode to compose images with the LCD Monitor and turn on Grid View so you can align the building lines with the grid on screen.
I think 22megapixels is plenty. Though, you may want more megapixels if you're making really huge prints. Other reasons for more pixels - if you have to crop out a lot of your image -like maybe you want a square image so you'd be wasting a bunch of pixels. Or, if you don't have a shift lens or can't shift enough and you have to do post-process perspective correction - that wastes a lot of pixels too.
