1) Sharpness is complicated.

Lens sharpness is just one aspect of the overall resolving power of a camera system. The appearance of crispness is separate from the rendition of detail. And a lens can be sharper in the center but not in the corners, or less sharp overall but consistent across the frame.

And that's not even getting into other factors that affect the system as a whole. Camera shake, subject movement, focus accuracy, diffraction — the list goes on. In fact, I'd go so far as to say that unless you are very meticulous with your technique, you won't ever notice a sharpness difference between these lenses, because any lens difference will be overwhelmed by the other things.

2) Sharpness is overrated.

It's fine if you like it, but recognize that sharpness is just one aspect of optical performance. All lens design is about compromise between various factors, and correction for one thing inevitably leads to compromise in another. See What image-quality characteristics make a lens good or bad? for a detailed look at this. Generally, though, when someone is saying that a lens is better than another even though the other is sharper, some of these other factors are probably at the forefront.

3) Don't forget that extra speed.

An f/1.4 lens can stop down to f/1.8. An f/1.8 lens can't open up to f/1.4.

4) But, finally, don't read too much into reviews — and don't get obsessed with "best".

There are basically three kinds of lens reviews on the Internet.

First, there are highly technical ones which measure the optical compromises and flaws using a workbench and special software and perhaps lasers. These are fun and can sometimes reveal useful information, but in general tend to emphasize small flaws and differences which rarely actually make a big difference to results.

Second, there are reviews by people who bought an expensive lens and need to tell everyone how great that lens is to help their internal justification. Since here we are comparing a $1700 lens to a $500 one — or a $200 one, if you are looking at the DX version — this may play more than a small part.

Third, there are subjective reviews from experienced photographers showing work they've made with a lens and talking about how it handles. These rarely mention technical details except in passing. These are the most useful in some ways, but you really have to read a lot to get a sense of whether you'll agree.

Overall, the best way to make this decision is to borrow or rent the lenses in question and try them each for a couple of weeks. (You need at least that time to really form a working relationship.) You'll probably find that you like the results you're getting with one better than the other — or if there is no clear winner, you can decide to go with the cheapest.

It's really easy to say "I clearly want the very best one!", but this is a trap. And, obsessing over technical reviews and measured sharpness play into that trap. What you want is a lens that fits your purpose — and your pocketbook.