In many areas of technology you’ll experience a variation on the “Law of Diminishing Returns”, or, the economic principle that you don’t get a direct resulting output for a corresponding input.  The input, in this case, is money. 

This is clearly the case with these two lenses, except the only two clear areas that you’re benefitting from your investment are in the speed of the lens- by picking up one full stop, and by having a nicer effect of the background de-focusing.  In every other respect these two lenses are remarkably comparable.  Granted, our MTF graph does show slightly better resolution as we work in the middle of the aperture range, but that’s not a particularly significant factor in actual use of the lens, and certainly not a clear advantage considering the drop in MTF at the f1.2 setting.

The conclusion is quite simple.  If you need that extra stop, and intend to use the EF 85mm f1.2L II USM at primarily full-open, then you need this lens.  You’ll get slightly less falloff (a questionable advantage in a portrait lens) and it will be on-par with the EF f1.8 USM in almost every other respect, but don’t expect your $2200 investment is going to buy you an across-the-board superior-performer.  You will be nicely surprised by soft, velvety feel of the backgrounds, but keep in mind that the depth of field at f1.2 is challenging (at best) for a portrait, unless, of course, you’re interested in a very selective focus on the subject’s eyes or the rose’s petals, for example.  The Bokeh Effect of the 1.2 is arguably, negligible if you intend to shoot at mid-range apertures. 

If you don’t need the extra stop, and frankly, with the increase in high ISO performance of any of the recent DSLRs, the additional cost and weight becomes even more questionable, then save your money and get a solid value at 1/5 of the cost.