The 50mm 1.8 is a very fine lens for an unbeatable price. I got one myself and am very happy with it.
What you should consider, is that this lens is not built very stable. I read a lot of horror stories about breaking apart after a couple of days. Mine still works, but the 1.4 is definitly better quality and it is a bit faster.
One might say, there is no big difference between 1.8 and 1.4, but if you’re shooting lowlight, the small difference might be crucial whether to have to push ISO (and noise) up or not.
Also: The AF of the 1.8 is very slow and not too precise and manual focussing is also easier with the 1.4.
There is no big difference in picture quality (I would say, there is none, but some people I know claim, they can see one…).
To cut a long story short: If you can/want to afford it, get the 1.4.
But I think, you also won’t be disappointed with the 1.8