Mega Latios Benefits
- Being able to run a Dragon Dance set effectively
- Higher attacking power for no Life Orb recoil.
- Knock Off is weaker.
- Great defences considering its Speed and attacking power.
I personally think Mega Latios is the worst of the two. Even though it is quite a good Pokemon, it's simply not a worthwhile Mega to run due to what you said in being able to get as much power with Specs or LO.
Mega Latias Benefits
- Incredible bulk allows it to set up easily. (80 HP/120 Def/140 SDef)
- Still hits quite hard with base 140 SAtk even without setting up.
- Knock Off is weaker.
- Still quick, meaning you can quickly gain extra bulk out of Calm Mind without having to worry about them hitting first.
- Incredibly hard to kill if played correctly.
Mega Latias is simply the better one here. It pretty much is one of the deadlist setup sweepers in the game aside from Salamence and Diance right now. Despite Latios still having a fair amount of bulk, it's simply not enough to let it stay around for extended periods of time compared to Laitas. In fact, I'll go as far as saying that you should only use Mega Latios if you want a Mega Dragon Dance user, but there also is better options there like Tyranitar or Gyrados.
