f/1.8 is 2/3 of a stop slower than f/1.4. In terms of shutter speed that means 1/160th instead of 1/250th. f/1.8 is still VERY fast!


DoF: Assuming a subject distance of 10 feet the total DoF @ f/1.8 will be 0.81ft vs. 0.65ft @ f/1.4.


Check out Bryan's reviews of these two lenses. They are both great in their own right. Since you are new to photoprahy and the f/1.8 is SO cheap I'd consider buying that and finding its limitations before upgrading to the f/1.4. It's a fantastic lens and probably one of the best ways to spend $100 to improve both your understanding of and quality of your photopraphy.