For code 83 track, my preference is Peco. Easy to work with, and looks good too. I purchased my track in bulk from Tony's Train Exchange.
I have a large amount of Atlas code 83 flex, especially on my helix. The only reason is because I had a lot left over from my previous layout portion that I ripped out. I don't think it holds gauge as well as the Peco, and it does not look as good. When I purchased my track about two years ago, the Peco was cheaper than the Atlas, and the Atlas was hard to get.
For switches, I've had good luck with Walthers over the years. I purchased Walthers again for my extension, and they are DCC friendly switches. I power my frogs thru the aux contacts of the Blue Point turnout controllers (manual) that I use.
It take a good look at the Peco Code 83 switches, if they were more widely known when I purchased my switches, I probably would have went with them.
Peco track and switches are VERY high quality.
Regards,
Jerry