At the moment with 'Big World' mobility is a key attribute to chase down opponents and win encounters. That doesn't mean to say that it will always stay that way. However, saying that - with all 'Open World' encounters mobility will still be a key attribute but there will be many objectives in the campaigns, choke points, POIs, sieges etc that mobility will not be as important.
So its not as 'black and white; to say that mobility is the best or it isn't. It will depend on the situation.
Also the ATs will never be perfectly balanced 1v1 given the same skill level of the players. To all those player who think it will that is not the design aim and I believe it will be impossible to achieve.
At the moment the ATs are broken and very unbalanced - that will change when ACE get round to all the other ATs that are not implemented. The ATs will be better balanced but you will not get perfectly balanced 1v1 fights.
Each AT will have their roles in group fights and that is where the skill will be. Choosing the right AT mix for a certain situation.