I've had the 50/1.8 and now have the 50/1.4. The 1.4 improves in a number of areas, but not by a great deal.
- Build is better, but the AF can get fracked if pressure is exerted on the front of the lens (perhaps in a bag or if knocked) when set to any focus distance other than infinity, so it surely no tank. The right lens hood offers good protection, but a cheapy which screws to the filter thread will do no favours at all for robustness.
- AF is a little better, but still poor, and not really up to the standards required for reliable shallow DOF photography.
- Of course it's 2/3 stop faster, but a little dreamy (soft) when wide open, so conceivably not offering the full advantage you might hope for.
- Bokeh is better when stopping down, avoiding the ugly pentagonal blur of the 1.8.
If it's only for occasional/amateur/casual use then I'd go for the 1.8 and put the money saved to good use elsewhere - e.g. a nice Nissin Di866 if you don't already have a flash. For the amount of use mine gets I might as well have kept the old lens. If you really want f/1.4 then consider the Sigma instead. So long as the AF is adjusted well I understand that it addresses all the shortcomings of Canon's effort. But that's even more money, and rightfully so.
I haven't voted, because a vote without an explanation is worthless, and now you have my reasoning explained the choice is very much up to you, not me.