December is really a great month as the weather is much more pleasant in this season.....unless you want a monsoon wedding.
Its basically more comfortable compared to summers which are unbearable even to locals. Besides, it's also the festive season so if you need to shop there will be plenty of sales and discounts, and if you need to party there's a lot of that too....
However, the west coast of India is a prime location for tourists...so make sure you do your bookings well in advance.
North India gets quite cold, and often there are cold waves...if you enjoy snowfall or extreme cold - go for the north. As from North India, they include states such as Rajasthan, Bihar, utteranchal, Jharkhand, UP, Delhi, Punjab, Haryana, Jammu and Kashmir, Chandigarh.
Don't know about the Gypsies tho - been in India all my life and none of the places you mentioned have 'gypsies'.
South India gets cold too but not as much as it is in the North... Mumbai or Goa are better in anycase considering their cuisine (you get a variety here that is unmatched), hotels etc...
In South India you can go traveling as in touring - but it's not a place where you'd like to party - it's more of sight seeing as there are beautiful temples and monuments....South India is also famous for its silks....people tour South India mainly for shopping.
If you are coming for a month or so, Visit Mumbai for a week, Goa for 10 days and then tour....
If you visit Goa, don't forget to visit the beaches and the churches....The Churchg of Bom Jesus has the body of St. Francis ...that one, and the churches in the temple complex opposite as well as St. Cajetan are amazing examples of Baroque and Corinthian architecture.... and the beaches with their shacks (serving sea food) and clear water are an amazing experience at night for candle light dinner. Resort Mello Rosa and others offer accomodations at varying rates to suit your budget....
In Mumbai, there is a lot of night life with salsa parties, discos, multiplexes as well as other tourist attractions like the elephanta caves, Kanheri caves, Mahakali caves, bassein fort, the Prince of Wales Museum and several art galaries. This is more of a cultural centre, a financial hub and houses the film industry of India popularly known as 'bollywood'. Though the beaches aren't as clean as those of Goa, it's a fun place....there is always activity - festivals, shopping malls, trade fairs(you can get stuff here from any country and any state or region),exhibitions , various cuisines, dance programmes, music concerts, dance etc.....and the city wakes up early so there is something to do at any time of the day....
Good luck with your reservations.....