I think you should chose Excadrill between the 2. Excadrill is actually super great, as it is an excellent counter to Tyranitar, abusing the Sandstorm for the use of one of his 3 wonderful abilities (unless you run Mold Breaker) and a spinner is always great to have. But actually, thinking wise, why don't you get both ?
If you can't decide, then you will have a great offensive Mega-Evolved Pokémon, and a spinner. A spinner is mandatory on teams with Dragonite, just because of the ability, Multiscale.
There are not Mega VS Not Mega.
Honestly, if I were you, I would simply try to get both, or at least Excadrill.
Excadrill is a spinner with a huge attack, access to Mold Breaker Earthquake and a counter to Tyranitar lead, if you have Swords Dance + Sand Rush and Earthquake to kill Tyranitar. Lucario, on the other hand, provides nice offense but is not so useful, except in double with Beat Up Scarfed Sneasel.