I think most of the answer for 'why a city law firm' will be similar to the usual 'why commercial law', but I would use one or two different sentences to reframe it and address the City element. I would focus on the City's standing as one of the world's leading financial centres and the corollary - that the biggest international commercial clients will look to City firms to advise on their complex transactions and disputes. City law firms will also tend to coordinate and lead numerous other offices and counsel from a variety of jurisdictions to deliver on their clients' mandates. Thus, if you are particularly interested in high-end and cross-border commercial matters, you can justify an interest in working in a City law firm as opposed to any other commercial law firm.