documentclass{article}
usepackage{filecontents}
% UseBibLaTex is a variable that if is less than 1 the document
% will have to compiled with `bibTeX`... else with biber
defUseBibLaTeX{0}
% ``url'' package is used in order to make LaTeX to break the urls
% in places like the bibliography combined with the next (upcoming)
% command
%usepackage{breakurl}
usepackage[hyphens]{url}
Urlmuskip=0mu plus 1mu

ifnumUseBibLaTeX>0
usepackage[backend=biber,style=authoryear]{biblatex}
newcommandmyurl[1]{#1}
DeclareFieldFormat{url}{{footnotesize URL:}space%expandafter%noexpand
myurl{#1}}
addbibresource{myBib.bib}
else
% ``natbib'' package offers the bigger ammount possibilities in bibliography
% formating
letmyurlurl
usepackage{natbib}
fi


begin{filecontents*}{myBib.bib}
@Article{cite1,
title={One Article with a url that has to break both in bibtex and biblatex},
author={Someone NotMe and Someother Me},
year={2018},
month={10},
url={url{http://A-long-URL-that-has-to-break-through-lines-and-biber-has-to-ignore-the-textbackslash url-command}},
doi={http://dx.doi.org/10.1038/nphys1170}
}
end{filecontents*}

title{Double Functionality about Bibliography}
author{Konstantinos Leledakis}
date{October 2018}

begin{document}

maketitle

nocite{*}
ifnumUseBibLaTeX>0
printbibliography
else
bibliographystyle{agsm}
begin{sloppypar}
  bibliography{myBib}
end{sloppypar}
fi



end{document}