Here's the current M-Latios set I've been using (I wrote a long post about it):
http://www.smogon.com/forums/threads/latios-latias.3496902/page-8#post-5839669
Honestly, the only reason I see using Mega Latios is if you literally want a Mega Defogger. He has more immediate power than his Mega Sister (who cannot break 400 Sp.Atk unless running a Modest Nature), and in my set, Latios achieves 437 Sp.Atk with 363 HP while reaching 279 speed (allowing him to outpace threats up to, and including, Jolly Taunt Heatran.) Basically, this M-Latios can Defog all day long with Recover, and can nuke stuff with Draco Meteor if necessary. You even hit a little harder than Life Orb Latias.
If you slap the set I posted on Mega Latias you're weaker by 31 points of Sp.Atk but marginally bulkier (the extra bulk doesn't really change what you're 2HKOed by so the extra power boost with Latios is more valuable. And remember, you don't have a life orb). With M-Latias you might as well just invest in max speed and sp.attack (reaches 379) and be able to speed tie with the normal threats.
I feel that Bulky Mega Latios is one of the best (if not
the best) defogger in the game. He's bulkier than his Life Orb Sister, Hits harder than her, and doesn't fear knock-off as much. He's
much more bulkier than his LO version at the cost of a power drop. But now the question is..."Is a Mega Defogger even worth the slot?. I suppose if you
really want Stealth Rock off the field, or if your team is based around a non-mega like Talonflame, Weavile, Greninja, Keldeo, Volcarona, Gengar, Terrakion, Thundurus, Crawdaunt, or Azumarill. But we all know these pokemon aren't built around much anymore, and are usually used to support Megas. So in conclusion, it's probably not worth the mega-spot.
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The Mixed DD set for M-Latios is heavily outclassed by Salamence at the moment. But even then, LO Latios hits harder than Mega Latios, doesn't require a turn to setup, and doesn't take up a mega slot. Also it isn't prone to Zen Headbutt misses. I can't really see DD Latios taking off anytime soon, especially since it's still prone to priority.