In your place and with hindsight experience of buying a bunch of lenses I would advice strongly to save a bit and get the very best first rate lens that you can afford later on. It is not worth spending some cash just because you have a couple hundred dollars to spare and coincidentally the 40 2.8 or the 50 1.8 are in that price range.
My advice to you would definitively be to save a bit and get yourself the Sigma 35 1.4 which used can be a very attractive proposition. The 6D deserves first rate lenses and the 35 1.4 (a top tier 35mm, sharpest of all) will serve you handsomely in moderately wide landscape, general and even portraiture. Not to mention super fast 1.4 aperture that will do wonders in low light with the sigma....
I went the other way. I would suggest not losing money when you buy and subsequently sell lenses. The best way to do that is to buy a lens when it is on sale (obviously), but even more so, buy used good condition lenses and canon direct refurbs.
So let's say the normal price for a 50mm f/1.4 is a $349. There was a sale price for $285 the other day. Or a canon refurb is $320 but with a 20% discount, it would only be $256 (before tax). It's subsequent resale value is around $300 in really good condition.
So buy the lens, use it for a year or two, and then sell it for what you paid, or maybe for a little bit of a profit.
The key is keeping it in like new condition.
So buy a lens at a good price, and use it for a while and then when you are ready to upgrade, it is almost like you had that money in escrow and you are able to tap into any time you want.
As an FYI, bodies lose their value quickly because they are replaced frequently with better models. Lenses aren't quite that way because even if a better/newer model comes out, it is still cheaper than the newer model which helps keeps its value higher, and more often than not, the older lens is still really good.
3rd party lenses lose value much faster than Canon lenses. They may be good, but the market for Canon lenses is significantly larger than Tokina/Sigma/Tamron/etc.