Go for the tilt shift lens. I have both the 50 1.4 and 50 1.2L. Image quality wise you can't tell the difference between the two unless you blow the image up and look really close, even then it's hard to tell. You can also see it in their similar MTF charts from Canon's site. I bought the 50 1.2L for its better build quality, better focusing, 1.2 aperture and mostly importantly my professional shoots that makes me money.
Since you're not in my situation, a tilt shift will give you much more FUN in return. 50 1.2L is not that much different than what you've already got with the 50 1.4. Don't give me wrong, I love my 50 L, but unless you're making money from photography, or rich, it doesn't justify 4x the price of the 50 1.4.