On the one hand, sirloin and tri-tip have a lot in common. They're both from the sirloin primal cut (with the exception of sirloin tip steaks, which come from the round). This means they're both from ...
Sirloin is a primal cut, further down toward the rump of the animal from the rib. Steaks cut from muscles get more work than ribeye, which also makes them a leaner choice for grilling.
For the chips, peel the potatoes and cut into about 1x7cm/½x3in chips - the most important thing is that the chips are more or less all the same size to ensure they cook evenly. Rinse the chips ...