depends on ur budget, if u have a budget of less then 800k then go for city exis. u ll get one in excellent condition. if u have more then 900k then go for city idsi. u CAN find idsi in less then 800k too but the condition would be bad so i wont recommend that.
as for pure comparison of both the cars, well, both have their positive and negative points. idsi is more economical, quieter, roomier cabin, bit more modern but city exis is faster, has better brakes, rides more stable and is simpler to maintain. but if budget was not an issue n i had to chose, i'd take the idsi...