The main difference is that top sirloin is boneless while sirloin includes a bone. Neither will be as tender as the fillet mignon, "Kansas City" or "New York" strip, or the ribeye steak.
"The loin yields the most tender and expensive cuts of beef--but not the most flavorful.* The choicest portion is the tenderloin, which is exquisitely tender and lean.*The top loin and sirloin aren't as tender, but they're a bit more flavorful.*Cuts from the loin require very little work to taste great.* Indeed, steak lovers consider it almost a sacrilege to marinate them, or to cook them beyond medium rare."
"Some top sirloin steaks are wonderfully juicy and flavorful but others are mediocre, so this is a risky steak to buy.* Don't confuse this with an ordinary sirloin steak, which includes a bone.* American butchers call a thick top sirloin steak a chateaubriand, although the French reserve that term for a much better cut from the tenderloin."http://www.foodsubs.com/MeatBeefLoin.html
"Full Beef Loin Untrimmed: This cut provides the choicest cuts of beef. It is from this full loin that the whole tenderloin is removed. Then working from the leg end, we first have the sirloin This sirloin (posterior portion of the full loin) provides the top sirloin roast which we use for roasting.This comes from the area just behind the flank, which is cut from the belly portion of the sirloin. As we move forward in the full loin we have the short loin which provides the sirloin strip from which we cut the New York sirloin strip steaks; the forward most portion provides the porterhouse and T-Bone steaks cuts."http://www.geocities.com/TheTropics/5245/content3.html
"The top sirloin. Also known as the top butt, round, or rump steak - you can see why I favor the name "top sirloin". This can also be found on menus as a London broil. This is great cut of meat with a really full flavor. However it is not as tender as the cuts mentioned above. While this is one of my favorite cuts, I don't sell it as a steak here in Japan because tenderness is an issue. However I do take this cut and make cubes which are great for stir fry, stews, and especially, shish-ka-bobs."http://www.themeatguy.jp/newse1.html
"Top Sirloin; this cut is a lesser grade but larger cut of meat. A family of four can eat from one top sirloin. Try to buy the top or prime grade, they will be more tender than the lower grades."http://ezinearticles.com/?Different-...Steak&id=43616
"Sirloin — This portion of the carcass provides boneless top sirloin steaks. If the cut includes the bone, various labeling includes wedge bone steak and pin bone steak. A steak labeled tri-tip or triangle steak has been cut from the less tender bottom sirloin."http://www.privateclubs.com/archives..._to_basics.htm
For a detailed explanation with diagrams on the cuts, look here:http://www.inspection.gc.ca/english/...beeboe2e.shtml
More diagrams and info:http://www.csuchico.edu/agr/grsfdbef.../beef-cuts.gifhttp://www.beeffoodservice.com/Cuts/Info.aspx?Code=45http://www.steakperfection.com/cut/Sirloin.htmlhttp://www.bigwoodsbison.com/Cuts.jpghttp://www.inspection.gc.ca/english/.../nomdese.shtmlhttp://www.ads.uga.edu/annrpt/1997/97_066.htm