Intriguing. It seems that both possess merit indeed, provided that Charizard X is used enough to merit running it over Adamant. Charizard Y has the chance to be dispatched in two +2 Bullet Punches sans SR, and sure as hell can't switch in to a CC + BP, but we'll definitely see.The following calculations should answer your questions. The calculations are, of course, tailored around Mega Gengar (using it in calcs since I predict Mega Gengar will become popular) having 80 Defense, and Lucario's other properties.
So with Stealth Rock damage, you're all set. You also gain guaranteed OHKO's on normal Gengar, which should answer part of your question. Obviously, you can't really say the same for +0.
- +2 252+ Atk Adaptability Lucario Bullet Punch vs. 4 HP / 0 Def Gengar: 250-296 (95.41 - 112.97%) -- 75% chance to OHKO
- +2 252 Atk Adaptability Lucario Bullet Punch vs. 4 HP / 0 Def Gengar: 228-270 (87.02 - 103.05%) -- 18.75% chance to OHKO
And for normal Gengar:
- +0 252+ Atk Adaptability Lucario Bullet Punch vs. 4 HP / 0 Def Gengar: 126-150 (48.09 - 57.25%) -- 45.7% chance to 2HKO
- +0 252 Atk Adaptability Lucario Bullet Punch vs. 4 HP / 0 Def Gengar: 114-136 (43.51 - 51.9%) -- guaranteed 3HKO
Somewhat unsettling considering Mega Gengar can't hurt itself with Life Orb recoil. Mega Lucario can also beat Gengar regardless because it naturally has more Speed than it, and uses Crunch.
- 252+ Atk Adaptability Lucario Bullet Punch vs. 4 HP / 0 Def Gengar: 158-186 (60.3 - 70.99%) -- guaranteed 2HKO
- 252 Atk Adaptability Lucario Bullet Punch vs. 4 HP / 0 Def Gengar: 144-170 (54.96 - 64.88%) -- guaranteed 2HKO
Targets should be highlighted in my previous post. Other targets include Garchomp, Salamence, Mienshao, and Charizard.
Salamence and Garchomp are certainly valid, though maybe less so due to the rise of faeries, but that doesn't make them not potent and definite threats to look out for. That said, 4/0 Garchomp will always live a Jolly CC from a +0 MegaLuke sans SR, and will live 2/3 of the time with it, whereas Adamant OHKOs about half the time sans SR, and a bit more than 80% of the time with it... Though you could, of course, just Bullet Punch - the two hits'll make you lose a quarter of your health in total, though.
Mienshao? C'mon. Unless I'm missing something, Mienshao is definitely a threat, especially in UU, but... You don't really see it in OU, and you'll probably see even less of it due to its two main weapons - High Jump Kick and U-Turn - both having been simultaneously nerfed due to the faerie type, as well as the second-hand effect of encouraging the usage of Poison types.
That said, Jolly's probably generally better, but extra damage can, of course, be useful. Whatever your team looks like, I guess?