I really don't understand these "comparison" threads. There are 2 fundamental questions to answer when it comes to buying a lens.
Do you need the lens in question? If yes, move to the next question, which is, can you afford the lens in question?
Most of the time, the answer to the first question is yes, and many people posting in these "vs" threads come here to justify the answer to the second question. Unfortunately, it just so happens that majority of these questions are asked to justify why the expensive version of a lens is unnecessary.
Of course 1.2 is better than 1.8, why else would it cost $2,000? Can you afford it? Do you need it? There is your answer.
An M3 is %30-50 more expensive (depending on the options involved) than a 335. A 335 is a phenomenal car to 90% of the public and a reach for many people. The M3 is a master piece, it's a car in a league of its own. You go to any public car forum and ask this question, 9 out of 10 people who don't own an M3 will say it's a waste of money and how you can get an 335 and chip this and mod that and beat an M3 0-60. Reality is M3 can never be beaten by a 335, because when you take the two cars, you never compare the 0-60 times. It's the suspension, the differential, the seats, the V8, the body work, and more importantly the name that makes an M3 the legendary sports car.
I cannot afford a 1Dx, but can only afford a 5D3. I don't go around saying how 1DX is a waste of money and how 5D3 is more than enough. Totally different cameras for totally different purposes. Now, even if I had the need for 1DX, I still wouldn't be able to afford it and stick with 5D3. That doesn't mean I'll try to justify my decision by saying how such a waste of money 1DX is. Makes sense?