With Cities, you either need to beat him or join him. If piles are unlikely to run out, you should just let him buy expensive villages one by one. Until a pile is empty, Cities are way overpriced, especially if there's no payload to go with the extra actions. With at least 10 turns to build your deck, you should be comfortably getting enough points so that by the time the City pile is empty, it's too late to come back. If you go this route, DO NOT buy any cities yourself, since that will only help to activate his sooner.
If piles are likely to run out (usually Curses/Ruins, or good cheap cantrips like Caravan), you should probably get some Cities yourself, and aim for at least an even split. +2 cards +2 actions is very powerful, especially when there's useful things to be done with all the free actions. With just a little extra drawing power, a souped-up City deck will have no trouble drawing the deck every turn. Be careful about emptying the second pile on your turn, since the extra $ and buy from fully powered Cities can sometimes allow your opponent to end the game quickly and unexpectedly. Ideally, empty a pile on your own turn, using a gainer (like Workshop) or Remodel variant.
The key with City games is to recognize which situation you're in before the game starts. If the game will go on for awhile, you probably want Cities with lots of endgame power. Otherwise, you're better off ignoring it completely and shutting the door before the Cities activate.