A rib of beef can come from the forequarter or front end whereas sirloin is from the back end or hindquarter, and is in essence a steak cut, part of a sirloin is the fillet of beef , bought together they are sold as a T Bone steak this cut should be more tender but this is dependant on the cooking method.
you can get a rib which is only a 'cut' away from a sirloin.
its called the 'wing' rib.
normally a set of ribs come in 3 or 4 bone. the wing rib is the less fatty end.....it can b dark (which is a good sign).
theres no such thing as a sirloin rib.....just the sirloin end (wing)
as solera sed, the sirloin is from the hindquater but it can be used as a roasting joint too as well as sliced for steak.
be careful tho nowdays theres lots of cow beef on the market being sold as rolled sirloin.....u cant steak it....well you can but it would be dreadful....
......btw, im a friendly butcher lol for 30 years !