Is the rails 4.2 web-console gem a complete replacement for the better_errors gem or do I need to look at the features of each to work out which one I will prefer?
They're not the same thing.. The web-console even recommends better_errors on it's README:
Check out better_errors as a great alternative for any Rack application!
The advantage of web-console is that you can start a console session calling debug anywhere in your files or views, pretty much like binding.pry (from pry gem) always did.
The better_errors is an improved error screen that happens to have a console session on the sidebar (if you use the binding_of_caller gem with it).
IMO, you can use both gems.. no need to choose one or another.