Man, congrats on the a6400! That body is such a beast for its size. I saw your post yesterday and had to jump in because I went down this exact rabbit hole a couple years back. Honestly, if you want something that basically disappears on your camera while delivering insane quality, you gotta look at the Sony E 20mm f/2.8 Pancake Lens. It is ridiculously tiny, like barely sticks out past the lens mount. I took it on a backpacking trip through Tokyo and it was a total game changer for street shots since it feels so unobtrusive. People dont even realize you have a camera pointed at them, which is perfect for candid stuff! If you want a bit more reach for portraits, I actually ended up grabbing the Sigma 16mm f/1.4 DC DN Contemporary Lens for a while. It is a bit beefier than the pancake, but the optical performance is just stellar. The depth of field you get at f/1.4 is super buttery and makes portraits look legit professional. I remember standing in a busy market at night and the light gathering ability just blew me away compared to the kit lens. It might feel a little bigger than what you want, but the results are totally worth the extra weight in your bag. Just my two cents from someone who obsessed over the specs way too much!

Adding my two cents here because I spent a small fortune testing lenses for my own a6400 last year and honestly, I kinda regret not looking elsewhere sooner. I spent way too much time obsessing over the Sigma and the Sony 35mm only to find that neither really suited my street photography flow. I had issues with the autofocus hunting in low light with the Sigma, and the Sony felt way too plasticky for the price tag, which was super frustrating. After that letdown, I tried a few other things that actually kept my bag light:

• Samyang AF 35mm f/2.8 FE is tiny. Like, basically a body cap, but it captures such clean colors.
• Sigma 16mm f/1.4 DC DN Contemporary is a bit wider, but the build quality is night and day compared to the others. Just wish it were smaller.
• Viltrox 23mm f/1.4 STM which was honestly a surprise performer for portraits. I really wish I had just grabbed the Samyang and called it a day. It is way smaller than the other options you are looking at and honestly, for street stuff, being invisible is better than having an extra stop of light. Don't make the same mistake I did by overthinking the specs and ignoring how the lens actually feels on the body. If you want something that you will actually bring on your trip, definitely lean towards the smaller glass.

Congrats on the a6400, it is a fun little body for sure. Honestly though, choosing between those two was a headache for me too. I had the Sigma 30mm f/1.4 DC DN Contemporary for a few months and unfortunately it just didn't live up to the hype. The sharpness is there, but the build feels kinda plasticky and I had issues with the autofocus hunting in lower light situations which is basically the worst thing for street shooting. If you want something reliable that wont let you down when you only get one shot at a moment, check these out instead:

• Sony E 35mm f/1.8 OSS
• This one is better because it has stabilization which is a total lifesaver for handheld city shots at night.
• Sigma 16mm f/1.4 DC DN Contemporary
• If you want a wider look, this lens is built like a tank, though it is a bit heavier than you might like. Personally, I ended up getting a used Sony FE 28mm f/2 and it has been my favorite carry-around lens by far. Its super compact and surprisingly rugged. The 30mm Sigma is technically okay, but imo its not as good as expected for the price. Just take your time, dont rush into it, and maybe look for a nice copy on the used market so you can save some cash for extra batteries.