I use the Peco Code 55, because I like the combination of the lower profile rail appearance, the ability to use large-flanged old rolling stock from my teenage years for which I have sentimental feelings, and the reliability and availability of choices in the Peco Code 55 turnouts. I realize there are better looking choices for the ties (i.e. Atlas code 55 looks beautiful).
I use Kato Unitrak exclusively after years of using Atlas, Peco, Micro Engineering and other brands alone and in combo's, but when I retired I realized I was not as physical as I was in past years, and standing up and re-railing a train especially in a remote area was very demoralizing. It is for pure reliability as since 2006, I have probably had 3 uncouple/derails max, and I lay careful track on every layout. It is this dagnabbit Florida weather and humidity!!!!