If I had a strict choice between the two, I'd get the 40mm lens. Why? First, it's cheap. Second, it's sharp. Third, it's 64mm on your camera which is pretty good. However, as RLPhoto hinted at, what focal length do you need? I would be happy with 64mm, but really the reason I shoot all FF is because when I buy a 40mm lens, I prefer not to deal with a crop factor for most shots. I have a 1D4 because I don't mind the crop factor for sports. So it really depends on what you need. Picking between your two choices though, I would personally pick the 40mm.
I realised I have missed out a lot of great lenses when looking for a lens to mimic the "normal" perspective on a crop body. Any other things that I should take note of?
This is just my personal experience but the three lenses I've liked a lot on a crop camera are:
Canon 35 2.0 - Nice sharpness on a crop camera, focus is loud and a bit erratic but since it's a fairly wide lens even if focus isn't perfect it's normally close enough. Right now our main setup is this on a crop camera and the 24-105 on the fullframe. Works pretty well and I think I bought it for around $270 refurb from Canon. Great for the price
Sigma 10-20 - Obviously we're talking about normal range lenses here but if you've never used an ultra wide angle then you might look into something like this as well. Instead of giving you something BETTER that you already have it will give you something NEW. The same could be said of any of the macro options as well
Tamron 17-50 non VC - This is a pretty good lens for the money. If you're looking for something better rather than something new then this is a good option. Usable at 2.8 and really sharpens well at 4.0.
I've used the Canon 50 1.8 and actually like it better for video because the focus ring is smoother than the 35 2.0 but for everything else I've liked the 35 2.0 better. Sharpness wide open can't compare and I think the 35 has better contrast too. The Canon 50 1.4 might be a little bit better but believe me I've researched it over and over again and I a have hard time wanting to upgrade. Being on full frame now I do miss the framing of the 35 2.0 on crop so I'm renting a Zeiss Makro 50 2.0 for a wedding in September. Looking at the charts, it looks to be much better than Canon's options. Of course it's manual focus only and not cheap but I'm using it in place of the Canon 100 2.8 IS macro I normally use so if it can meet both needs it might get rented again.
I've also researched the 40 2.8. Everything seems to be really nice about this lens. My main hesitation is I already have the 35 2.0 and I'm waiting a little while longer to make sure there aren't any issues with the new focusing system on the 40 2.8. Otherwise I think I'd recommend it over the 50 1.4 but that's based on the fact that I like my lenses to be sharp wide open mainly because of the fact that unless you have the camera in AV or M it's going to use the biggest aperture.