exkeks

I had both, but kept the 27mm only as my compact setup lens. On the other hand, 23mm is my preferred focal length, so I got the 23mm/1.4 for all the occasions when size is not crucial.

This stores the value you entered into the EXIF data. That can be useful if at some point in the future you want to remind yourself which focal length (lens) you used to get that image. Additionally, if your raw converter allows you to use custom lens profiles during the conversion process, this information already being in the data can save you some time by selecting the appropriate one for you. The lens profile can contain information regarding how much vignetting the lens has for a given f-stop, how flat or fish-eyed a lens is, etc. Many raw converters have some generic profiles they use for lens that cover a wide variety of settings. 
It does no harm to leave it empty, filling it in and remembering to use may give you some extra options.

What camera do you have? I've got an X-T1 and X-E2, with maybe the newest firmware (if not it's pretty close to).
 
I've got the 23 1.4 and 35 1.4 and the 23mm is definitely faster and more consistent. The 35 is pretty close in decent light, but in bad light it's definitely worse.
 
Now that I think about it, have you done the firmware update for the 35 1.4? It had an update a year or two ago that made the AF noticeably better. That plus whatever body/firmware you're using could make a big difference. 
 
Like I said, the 35 is noticeably worse than the 23, but I wouldn't call it painful, so I'm wondering if there are other factors involved here.

Perhaps you would have better luck selling them separately. More people might be interested in particular lens, or body only.

Yes that is what happens!