Mega Latios:
HP - 80
Attack - 130
Defense - 100
Sp.Atk - 160
Sp.Def - 120
Speed - 110
Mega Latias:
HP - 80
Attack - 100
Defense - 120
Sp.Atk - 140
Sp.Def - 150
Speed - 110
As you can see, Mega Latios is more offensive (sweeper) while Mega Latias is more defensive (wall). Both are capable of runnung opposite sets but don't do it as well as their other counterpart.
Source 1 (Mega Latias)
Source 2 (Mega Latios)
As for appearance differences, many people say they have spotted a slight size difference, different eye colour, although some say it's just due to flight angle differnce. All in all, so far there are none confirmed.
Hope I helped!
