the is basicly no difference between the 2 in new zealand, where i live, as we have a very high quality of beef and lamb products. but in other countries like America it my be different. The top sirloin steak would just be of a higher quality, so you could use sirloin steak, but there may be a slight difference in the "texture" of the meat. you may need to cook for less time or use more marinade if you are using marinade, ect, to regain some of the juicyness back to the meat.
Kathy, Northland, New Zealand