The fat % of raw hamburger is not nearly as important as many think. The fat % of the final product is what is important. As touched upon above, the cooking method is the most determining factor in the fat % of the cooked product. Regardless if you want the final product in a patty or crumbled you can drain the fat off as many times as you choose to lower the fat content.
Raw ground beef fat % is as follows:
Ground Hamburger: 30%
Ground Chuck (Shoulder): 20%
Ground Round (Hip): 15%
Ground Sirloin: 8%
Any of the above ground meats could have virtually identical final (after cooking) fat % if fat is repeatdly drained.
Using a "London Broil" (Top round roast or upper hip) would only yield a slightly lower raw fat % than the standard round 15% because it has most likely been trimmed more than what would be used in standard ground round. But the meat is identical (round).
Economics should IMO be the major issue at choosing which ground beef to buy. For instance, 5 lb. of ground HAMBURGER (30% fat) = .7 * 5 = 3.5 lbs. of meat. On the contrary, 5 lb. of ground sirloin = .92 * 5 = 4.6 lbs of meat. Therefore, ground hamburger could be 75% the cost of ground sirloin but you would actually be getting the same amount of meat/$. Another note is that it is worth something to reduce work of draing fat while cooking. For example, if ground chuck were $1.99/lb. and ground sirloin were $2.29/lb you would be getting the same deal in meat per $. Whatever additional cost the reduced work in cooking is worth you can add to determine how much more the ground sirloin is worth to you.
One more note. In the US most of the ground sirloin is derived from the cutter cow market. Since most of you don't know what a cutter cow is... it would be synonymous in humans as an 80 year old dried up woman. It is not most profitable for USDA Choice grade sirloins to be ground up into hamburger. So another point to remember when buying ground hamburger is that most likely the ground hamburger, round, or chuck is from slaughter steers wheras there is a good chance that the ground sirloin is from the dried up cow
My choice is generally always ground round.