SpecsMence has its advantages over Latios - most notably Intimidate but also the lack of a Pursuit weakness and a key U-turn resistance but most of all he has much safer spamming moves. Fire Blast is fairly safe in most situations whereas Hidden Power Fire is going to result on you getting set up on by 90% of the tier. Surf isn't quite as much of a liability but Hydro Pump is a far safer move to lock yourself into. When I'm using Latios I'm just spamming DM and I'm generally reluctant to use my coverage moves whereas with Mence I can fish for the Steel switch-in while still doing considerable damage to anything else that might try and jump in on that 525 SpA 120BP move.
Specs Latios sucks and has done for most of the generation tbh, you either spam a Dragon attack or gamble with that 70BP attack which even then seldom provides a favourable risk reward scenario...sure you take that risk and it pays off and you nail that Jirachi switch in for 44%~ but then Jirachi just switches out to virtually anything - Salamence can play the same gamble (and it's nowhere near as much of a gamble by the way because much fewer things can switch in on that Fire Blast) and smack the pixie for 73%.
Choice users are all about safe spamming and risk management while preferably offering some team support and defensive utility and you'd be hard pushed to say Latios outclasses Mence at that.