If you are serious about Real Estate photography, which goes in hand with Architectural photography, the lenses you're considering are not, by a far margin, a good choice. Your only choice is a Tilt Shift lens as Bob said. Canon makes 5 different ones, the newest ones are very expensive ($2,500) but offer a sharpness that is actually equal or superior to the 14mm L lens. Any wide angle other than a Tilt Shift will distort the perspective making the room look much bigger than what it is and will converge the lines on the side of the field. The 14mm L is a very expensive lens and quite frankly a waste of money on the XT.
Remember you can rent a Tilt Shift for about $45/day (cheaper by the week) and you can also use the 1.4x and 2.0x power converters with them.
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