Mega Latios, or Soul Dew Latios?

Soul Dew is definitely the better option. The only thing Latiasite has over Soul Dew is higher defense and attack (which you don't need). Soul Dew on the other hand, allows you to achieve a higher Special Attack stat and Special Defense stat than Latiasite does, and that's the main reason why it's the better item for Latias. It's basically a free Calm Mind

But isn't Soul Dew banned on the Battle Spot?

EDIT: Crap, I just realized I was talking about the wrong thing xD But yeah Latiosite is your only choice because Soul Dew is banned.

 

Soul Dew = Broken Latiosite = Crappy Life Orb

 

Soul dew vs mega stones is essentially asking 'free CM boost with a chance for it to be knocked off? or mega up for permanent stats?'
Easy answer. do the former. A +1 spa/spd is much more reliable than wasting a turn for a boost.

However, mega latios does have respectable attack (less so in ubers) if you want to run a dragon dance set, and has a great movepool.

 

If only it weren't timid! I'm the kind of guy who explores every possible set for a Pokémon, and a physical Latios is actually viable through M-Evolution. M-Latios may still have a sp. attack that outshines its physical, but it still has a decent physical attack stat there, as well as having access to a whole range of physical moves.
In short, a special Latios works better with Soul Dew, but a physical one works better mega-ed.

 

well, soul dew latios/latias is banned!(uber)
That means that this Pokémon with this item is super strong!
So i think soul dew is for sure the right option!
Soul dew=(+1 calm mind latios/latias)!!!!!

 

Mega. Mega by long shot because I never got to use Soul Dew

 

Soul dew is way nuch better.
Calcing which item gives a better boost, the soul dew boosts 120 base stats.
Obviously the mega item only gives 100.
120 > 100 so... soul dew is better.

 

Soul Dew is the far better option given what Latiosite changes stat wise. There's not nearly enough boost to defense or HP needed to make the item very viable, and you are locked into that mega. Honestly, here's how items go for Latios:

Soul Dew > Life Orb > Choice Scarf/Specs (given it has Trick as fodder) > Latiosite