The interesting options are :
- Canon 40 f/2.8 STM: maybe not wide enough, but very compact, very lightweight (130g), and quite cheap. But I have seen reviews that criticized the autofocus and the overall image quality.
Between the Canon primes, what is the best lens in term of image quality ?
I suggest you ignore that review source. The reviewer clearly couldn't get his head round the price. They have probably measured a tiny fraction of increase CAs and found the STM is slower than USM. Big deal for the intended use of the lens. <sarcasm>. I find www.photozone.de
to give reviews that match what I find in practice with all my lenses so far, despite the fact that they only test one copy of each.
If you are referring to the lenses you have listed in bold type I'd say they are all virtually identical. The 40 Pancake is a great little lens but, if you are intending to get the 35 IS I wouldn't get the pancake because although 40 is noticeably wider than 50 - especially close up, it is quite similar to 35mm.
The 40 goes well with a 28 or 24. Personally I am more of a 28 fan than 24. True, that focal length is certainly out of fashion, but I find it more versatile than 24. It's also cheaper, and i am the ultimate photographic cheapskate