Dear Alvina first of all both r different feilds...but here if u make comperison then CSS is a good choice... and also AD isi is good for those who aint selected in CSS... but its not good that u prefer upon CSS. Ad ISI is tough job for ciclivain i have asked some officers in their..and they were somehow tired of it...they said that we works under Army officers and they dont give such values to civilian officers and mostly they get degraded and taking hard work from them...some officer said they will try to shift to FIA when post will announced in FIA. so now u can see ur self...

so now Good choice is >> CSS r FIA....

Regards,
Asif