Steam locomotives come in all shapes and sizes, this is because their intended tasks require varying traction...
It is difficult to say whether Peco or Hornby is better as it depends which criterion you are looking at.
Peco sleepers are brown while Hornby are black. Which one is best is a matter of personal preference.
If you want code 75 or electrofrogs, then the only option is Peco. The range of points is also larger from Peco.
Peco is a company that has been manufacturing track for the past 70 years while Hornby is more focused on making engines and rolling stock.
The Peco points seem to be manufactured to a closer tolerance, meaning they are likely to be more reliable however lots of people use only Hornby points with no issue.
Most exhibition layouts seem to be using Peco.
If you buy a Hornby train set, it will come with Hornby track so you may want to continue with Hornby track. If building a layout from scratch you may want to experiment with another brand of track. Bear in mind, Peco and Hornby track are compatible so you can mix and match and experiment as much as you want till you know which one is best for you.