I mostly fly Delhi-Bombay in business class.
Jet is a good option due to the sheer number of flghts - there is one every 2 hours or so. The call centre is surprisingly efficient and easy to get through. The lounges are fine(more spacious than Kingfisher) so second hand smoke is usually not a problem. Check in is usually quick. After this point though Kingfisher wins hands down.
Kingfisher has a limo-bus(if there’s such a thing) thats very comfy and stylish. The aircraft are newer and look smart, clean and spacious. The service is better, crew seem more experienced than Jet and smarter. Good menu options and entertainment system, etc.
I was hooked on Jet and their frequent flyer program till I had one really poor experience in August where almost everything went wrong from their reservation office booking me on the wrong flight, to a chaotic check in experience, shabby aircraft, poor service on board, etc. I complained via email and they reverted a week or two later to say the email had been passed on to various departments. However there was no offer of any kind of compensation/service recovery. I advised them I would switch to Kingfisher and have been true to my word. Incidentally they never responded when I told them this.
I would probably fly them again - simply because of their wider network and convenient schedules, and because they are a reasonable alternative for destinations Kingfisher does not yet serve.