What is your planed usage for this lens?
I have the 50 /1.4 but not the 40 /2.8.
However when it comes to DoF, the 50 /1.4 is a (slightly) longer lens and two full stops faster, that does give quite a big difference. You would notice. The 40 has less distortion BUT the wider angle causes problems with tight portraits. I have no idea if this is how you want to use the lens, but it´s kind of a warning.
My 1.4 however is seldom used at f 1.4 - 2.0 because of
- lack of contrast (and sharpness)
- DoF is thin and hard to controll and get the sharpness where I want it
- For weddings DoF is too short for portraits of bride + groom. For a single person portrait, yes, its quite doable, but when they are both there, better step down to... 2.8. And then they play in the same league.
Since I cant comment on the 40, you´d better read some tests at photozone.de and/or the-digital-picture. Have a look at their test shoots to compare contrast and sharpness, they are very useful.
But a quick look sugests that there is not much of a difference in terms of sharpness when stopped down a bit.
And... the 40 is quite a bit cheaper.
Im sorry that I perhaps did not make it easier for you... but I would not use the 50 /1.4 @ 1.4 unless absolutely needed, to me it´s a f/2 lens. And as I said, I usually use it even further stopped down, so then they are about the same, but one is cheaper.