If this is your SECOND lens, you'll be in heaven with the 1.8. It is true that the 1.4 is a little better, but it is geared for a more professional audience, or at least an audience that isn't as financially concerned. And despite what others say, for most its not three times better than the 1.4, even though its three times as much. In the lens world, they really rack up the prices for things that are towards the edges of photographic capabilities, like the telephoto primes, low f/stop lenses, macros, etc.
You're going to be in heaven. I don't have one, but I did on my dad's old film Nikon, and it really is cool. It won't make you a better photographer, but it will give you more options, and you'll love that first time you're at a Birthday party indoors and don't have to use the flash. Once that happens, you'll never want to go back again.