Factually speaking, the 50mm lens is the easiest lens to design and manufacture sharp. Any lens maker that cannot make a decent 50mm lens would be out of business.
Having said that, I know for a fact and by owner's experience that both lenses are sharp. I haven't tested them both side by side. Although I have them at my work place to do such a test, I don't see the need to because the images shot by them are good enough to convince me.
I once owned the 50mm f/1.8, and although I'm not a heavy and abusive user of it, the inner barrel began to wobble due to wear and tear. Their "expiry date" comes when you see the inner barrel wobble and droop to the pull of gravity. (Image quality becomes compromised and it's not worth servicing.) It's really not made to last, and I consider it to be a use-and-dispose lens.
A wobbling and drooping inner barrel was why I sold it off to buy a 50mm f/1.4. Apart from the advantage of USM (faster, quieter AF), it is obviously better built and more durable. Sharp it is, but it is also not perfect, just like its smaller brother.
If I remember correctly from my resolution test pattern at work, the f/1.8 lens exceeds 120lpmm but below 160lpmm in the center. (That's very good, and it matches or exceeds some popular L series zoom lenses I have seen.) However, like most lenses, it has resolution fall-off at the edges to roughly half of what is in the center. I won't be too anal about it because many consumer grade zoom lenses are still lesser performers than it. The chromatic aberration (CA) and flare at the edges are, on a lens test chart, very visible, and the f/1.4 lens isn't spared from this flaw too. Both lenses have them.
I know this topic isn't about 85mm f/1.8 but if I was to select and demonstrate an "excellent" lens that has very good resolution from center-to-edge, lesser CA, and lesser edge flare, the 85mm f/1.8 is the lens I would use. When I saw the resolution test pattern projected through this lens, it was obviously better. It makes the 50mm lenses look like sub-standard.
Generally, the principles of optics is such that the bigger the aperture, the more optical flaws to the lens, and greater effort is required to minimise or eliminate them. In all lens brochures, you seldom see words like "eliminate" because truthfully speaking, it is too difficult and too expensive to achieve perfection.
Having said all this, the real question to ask is whether or not we really need the larger aperture. How often do you shoot at f/1.4? Or f/1.2? Or if you can still find and buy the f/1.0? Also, keep in mind that the larger the aperture, the shallower depth of field is more difficult to work with. Wide open apertures tend to produce soft images partly due to flare and other optical flaws (which most manufacturers only minimise but not eliminate). The 50mm f/1.4 is soft and milky wide open but the smaller brother isn't (or isn't as bad). Truthfully, ask yourself if you're buying to show off?
I have the 50mm f/1.4 but I have not shot at wide open. I'm not inclined or motivated to shoot at f/1.4 because I know it's difficult and it's soft. (The 85mm f/1.8 isn't soft wide open.) There's no point stressing over images that are difficult and soft so I shoot at widest f/2.2 or f/2.5, and still very nice.