I'd say go with the 1.8. I'm not sure how the Nikon build quality on their 1.8 is, but if it's anything like the Canon there's definitely a lot more to be desired.
That being said, the reason I say go with the 1.8 is that it's a great deal for a great prime, and if you are careful with your lenses you probably won't be replacing it. 1.4 is nice but I guess it also depends on what/where you are shooting, since that's a pretty shallow depth of field. To be honest I don't use apertures larger than 2.8 on my 50 unless it's really low light. You could also up your ISO if you're really pushing shutter speeds to their lower limits. Considering you got the D300s, I don't think you'll be complaining too much with low-light shots
