Let's do the calculations. What would do more damage, a Choice Band Lucario or a Mega Lucario?
Let's do these calculations on a Level 100 Druddigon, where a Close Combat will be of neutral effectiveness.
Adamant Nature, 252 Attack EVs, and 31 Attack IVs for all Lucario calculations.
Bold Nature, 252 Defense/HP EVs, and 31 Defense/HP IVs for all Druddigon calculations.
Lucario w/ Choice Band - 220-261 | (61.4 - 72.9%)
Mega Lucario - 240-284 | (67 - 79.3%)
Mega Lucario would do 20-23 more Damage then a Lucario w/ Choice Band.
Both, however, are a guarenteed 2HKO.
Also, with Lucarionite, Lucario has the option to run mixed sets with its base 140 SpAtk stat in its Mega Evolution.
Lucarionite is better than Choice Band on a Lucario.