Stefano,
Depending when you want to depict 503 they operated in Russian and Normandy, 505 spent all its time in Russia again depending on which front and time frame you wish to depict depends on the Tiger latter variants would be the steel wheeled version or the Mid version a very good book to start with would be “Tigers in Combat” Vol 1 or if you are wanting a free reference the only place to look on the web is here
http://www.tiif.de all tiger Abteilung are covered on this web site if there is a particular picture you wish to model let me know where it came from or a copy of the picture and I may be able to tell you a little more