Generally speaking, the leaves on a Yucca will be nasty, sharp, pointy beasts which will take your eyes out. The flowers they produce are huge spikes with 2-3" flowers all up the spike...they are pinky-white. I suppose they could get to 12 foot at a push. They flowers usually get smothered in black fly.
Cordylines have much thinner, strappier leaves which which tend to bend easily. They are more then capable of growing over 12 foot tall either as a single or multi-stemmed specimen.
They produce a huge cluster of tiny white flowers which are very highly scented...some people say sweetly scented, others don't but they are very strong.
Both Yuccas and Cordylines can benefit from a good chop down....all the side growth, as well as new shoots from the stem may grow and take the main stem's place over time.