Originally posted by Tingchaleun ... [E]ven though the 50 has larger aperture, i don't recommend stopping down that much. From 1.4, up to 2.8, the photo is very soft. So, you might end up not using that end anyways (as i have ended up doing). The max aperture i use on my 50mm is 2.8. If your primary concern is opening the aperture for low light, then i consider them the same because I wouldn't be happy with the soft photos i get at larger than 2.8.
I hear this every so often about the FA50, and I disagree. We all have different opinions of what qualifies as "soft," of course, and maybe your copy of the lens performs differently than mine. But I believe that most of the time when someone claims that the 50 is "soft" below f/2.8, s/he is really complaining either about (1) the extremely shallow depth of field when the aperture is so large, and/or (2) a very slight front- or back-focus issue with the lens, which of course becomes more visible at large apertures with the shallow DOF. For what it's worth, I don't find my 50 "soft" at all.
Yes, I too try not to use the 50 below f/2.8 or f/3.5 unless I have to -- most of the time an extremely shallow DOF isn't particularly useful. But I certainly do not consider the 50 "the same" as a lens with a max aperture of f/2.8 -- that, in my view, vastly misstates / understates its capabilities. There's a huge difference (of two aperture stops) where the 50 is usable -- even if not ideal -- and the 40 is, well, not.
As for the OP's original question, the 2 lenses are clearly different, each with its pros & cons. To me, it's simply an individual's choice as to which set of features and compromises -- regarding price, size / weight, max aperture, build quality, and rendering quality -- to accept and which to reject.
Personally, I chose the 50, and don't regret it, but now am very very tempted to acquire the 43mm Limited. (And please, will someone tell me it's perfectly OK to have both the 50 and the 43?)