I can recommend keeping the 40mm. It works well as compact body cap that still has your camera in a ready-condition. Additionally, you may always find situations where you don't want to risk your nice 35mm to the elements you're shooting in. It's a cheap investment that you've already made.
Agree completely. The 40mm f/2.8 pancake "lives" on my 6D. Small, light, & minimum risk.
Another really fantastic little lens that I use a lot is the Voigtländer 20mm f/3.5 pancake. A little "kit" I often carry instead of my 24-105L (which has the "shorted ribbon cable" problem, awaiting repair) consists of the 20mm Voigt, 40mm pancake, and the 85mm f/1.8 USM. The 20/40/85 lengths complement eachother well. Any two of them will fit comfortably into a grey Canon "LP1219" lens bag. I have another lens bag that I use for accessories (handy to be able to take the two of them out quickly at security checkpoints too). I just chuck the two of them & the 6D in a neoprene case (usually with the 40mm mounted) into whatever bag or backpack I'm carrying at the time & go.
BTW the diminutive 270EX II also fits into the category of small/light/cheap; I consider it a must-have if you've got a camera with no flash like my 6D. I keep it loaded with lithium batteries as they last forever & don't fade.