Here's a picture of a Jovibarba heuffelii 'Xanthoheuff' flower cluster. Most of the heuffelii I've seen flower tend to do a (spherical) flower cluster at the end of the flower stalk, while sempervivum will often set flowers all along the flower stalk. Don't know if this is because of conditions or is due to the type of plant, but worth paying attention to.
BTW, I can't recall ever seeing my S. 'Oddity' flowering. Does anybody know if this variety can/does flower? If so, it would be interesting to collect seed to see what results and , if so, consider the possibilities with cross-pollinating with various other semps!