Mega Gengar
- higher attack (Sp. Atk)
- better defenses
- good move pool
- low HP
- more weaknesses
Mega Gengar is a really good Pokemon when used in the right way. The best way to use it is in doubles/triples, but singles work just fine. It's ability, Shadow Tag, prevent the foe from fleeing. Plus, Gengar can learn Perish Song, which you can either sacrifice yourself for 3 turns, in which Mega Gengar and the opponents will all faint (if still alive), otherwise, have it learn Mean Look so you can still save yourself (better having allies learning moves like Mean Look). Also, it can learn great STAB moves such as Shadow Ball, Dark Pulse, and Sludge Bomb. If you decide to use a Hypnosis strategy, have it learn moves like Nightmare or Dream Eater; and you can even have ally Pokemon such as Darkrai who know Dark Void, otherwise stick with a Pokemon like Breloom who know's spore.
Mega Lucario
- higher base stat total
- very balanced
- less weaknesses/more resistances
- good move pool
- slighty less defenses
Mega Lucario is a very tough Pokemon to go against, especially with it's ability Adaptability. With this ability, moves like Power-Up-Punch can help trigger the ability, and raise Atk at the same time. Otherwise, go for stronger moves like Aura Sphere which is useful in triple battles. Lucario is good in both Atk/Sp Atk, but it seems like it learns more physical moves - so a move like Swords Dance + STAB (with ability) will sharply increase it's Atk - otherwise stick with Power-Up Punch. Other great STAB moves like Flash Cannon is good, or consider using a priority move like Bullet Punch.
Overall, I think both Pokemon are equally good; so it all depends on the scenario. In my opinion I would chose Mega Lucario > Mega Gengar in single battles, but Mega Gengar > Mega Lucario in doubles/triples - but each of them can work in the other battle modes just as well.
Hope I helped! :)
