First, I think you will be very happy with Silk Road, a very hardy lily, proven performer, and with all the attributes you mentioned. Mine generally exceed 6-7 feet. Give it some time, the first year don't be disappointed if it only gets to four feet. It will tower over you in no time. If interested, read the sticky "adventures in scaling" about "silk road" propagation gone amok at the start of that sticky.
As for the difference between daylily (Hemerocalis) generally have tuberous type roots, where regular lilies have bulbs (think garlic), there are other differences as well, but that is the quick answer.
Hope it helps!
