This was already answered, but I feel like answering it as well :P
Before I start I need to say one thing
Neither one is better, they are each good in their own situation.
They are very similar, Latios has slightly more Sp Att and latias has slightly more Sp Def, but that is quite minor.
One of the biggest differences in them is their mega's. Mega Latios has more Sp Att and Attack, while Latias has more Defenses.
It really depends on the situation and the need. If your looking for a straight out attacker, generally latios is a better option, but for a setup Pokemon or for calm mind generally latias is preferred.
One thing I would like to point out is that Latios's extra attack isn't completely pointless. You can try and trick your opponent by running a physical latios. He has decent physical moves with EQ and Zen Headbutt, plus body slam.
I hope this helps and if you have any comments or questions please ask or say!