The price is dropping because the new 50mm 1.4 II (probably with IS) will be announced. I'd rather be patient to see the price point on the newer lens (which will be high).
On crop sensors, 50mm is too long. Get the 30mm for a general walk-around. For portraits, get the 50mm.
And whether to get the 50mm 1.4 vs 1.8, well I would definitely say 1.8 for performance/price. One hardly ever shoots at anything below 2.8 and at that aperture, both lenses are identical. Even at 2.0, they are pretty much the same (the 1.4 is slightly better). If you think you'll need 1.6 (cause thats the only f stop available that the 1.8 doesn't have, because using 1.4 is impossible, the lens is too soft), then get the 1.4. But, you will more than likely be using 2.8 on either lens, which makes the 1.4 pointless. If you need to stop down, go to 2.0, but thats more than enough DOF on a crop with this 80mm equivalent.
Think about it:
-you probably need a DOF of 1ft to get a face all in focus (forget group shots, just a portrait)
-using a 50mm on a crop at 2.8 standing 7 feet away from your subject, yields a DOF of 0.63 feet
-using a 50mm on a crop at 1.4 standing 7 feet away from your subject, yields a DOF of 0.31 feet
-you either need to step further away to get more of your subjects face in focus and crop in post, or increase the DOF by using a smaller aperture
-at this distance, you will more than likely use 4.0 and above, rendering the 1.4 useless
-and, if I were to use 1.6 and want a depth of field of 1.05 feet, I would need to stand 12 feet away from my subject. Ask yourself, how often do you do that? Is that even possible in doors? How would the shot look when the persons face is so small etc.
In the real world, there are very limited situations you can actually use 1.4 or 1.6. For a lens that costs 3X the price, definitely not worth it. Where 1.4 would shine is on a wide angle lens when you want a lot of background in your composition, but a very narrow DOF, like a model in a field. You want the field, but you want a razor thin DOF to get only her in focus with the rest blurred out. Thats why if you are looking for a 30mm 1.4, the new Sigma Art coming out this week will be fantastic.
There's some good info here, but I'll disagree on the general idea that fast lenses aren't very usable wide open. Just because you don't shoot very often below 2.8 does not mean that no one else does. It's more about your personal shooting style and the conditions you shoot in (f/4 indoors is a bad situation unless you've got great light or are providing your own). Specifically, for my 3 50mm lenses, here's how often I've used the apertures below:
50mm 1.8 - 48% of all shots at 1.8, 74% at 2.5 or below
50mm 1.4 - 30% of all shots at 1.4, 50% at f/2 or below
50mm 1.2 - 52% of all shots at 1.2, 82% at f/2 or below
These are only shots that were kept, not shots where focus missed or only someone's nose is in focus.
Based on the numbers, it looks like we have very different shooting styles, however many of my shots are at 2.8 - they're just taken with different lenses. For indoor/low light shooting, though, these fast lenses are great to have. If my zooms were faster than 2.8, I would likely be using wider apertures with them as well. I'm not sure if you've had a chance to use a lens like an 85 1.2 (or the Sigma 85 1.4), but there's a reason that people pay a premium for fast lenses.
If you're considering this lens, take a look at some samples on Flickr to see if you like the look/DoF @ 1.4:
50mm @ 1.4 on a 5D II: http://www.flickr.com/search/?q=p...=50&xfh=50
50mm @ 1.4 on a 7D: http://www.flickr.com/search/?q=p...=50&xfh=50
Not to mention, the wider aperture is not the main advantage of this lens over the 1.8. You get better/faster AF, a usable focus ring, full time manual focus, and a huge jump in build quality. If none of these matter, then you can evaluate it on the optical qualities alone.
As for waiting for the non-existent 50mm 1.4 II, as you mentioned, it will be significantly more expensive than this lens. Also, at this price, you can buy this lens, use it until the new one is out, then sell it for the price paid (or close). I agree that the 50 1.8 is the best bang for the buck from a price/performance perspective, but this lens provides some significant upgrades and shouldn't be overlooked at this price.