As you know nalidixic acid was the first quinolone to be introduced decades ago, then came norfloxacin, ciprofloxacin etc. Each new quinolone is an improvement over the preceding molecule in one way or the other. Zyquin should normally be used in community acquired pneumonia, acute bacterial sinusitis, acute exacerbation of chronic bronchitis, complicated urinary tract infection and certain selected cases of gonorrhoea. It has low proteinbinding and high tissue concentration properties. Also the possibility of photosenstivity with Zyquin is quite low. It is taken once daily.
Unfortunately each older quinolone has been used, misused and overused in India leading to resistance. They have been irrationally combined with imidazoles leading to widespread misuse in diarrhoea (even of viral origin) and development of resistance. Under such situations gatifloxacin has a role to play as initial treatment in those cases suspected to be resistant to older quinolones. Besides it should be preferred in hospitalized patients who need anti-bacterial cover because the possibility of resistance to older quinolones, particularly in critical care (e.g. ICU). There is theoretical possibility of cross resistance among all quinolones but some micro-organsims resistant to older quinolones remain susceptible to gatifloxacin.
In terms of cost, please keep in mind that older molecules are bound to be cheaper because the research and development costs have been recovered over long periods. The per day cost of gatiflozacin is about Rs. 60.