You should have no problem getting cheap flights to numerous Florida cities from Atlanta - you'll have lots of options.

Florida would, or course, be a much different type of vacation than Boston or Philly.

I have lots of family in Philly, but between the two, I have to say quite honestly Boston is my favored city,

I've lived in both Phila. and Boston. They'll both be nice in October, but Phila. may be a bit more pleasant and warm that time of year. Other than that, Boston gets the slight edge, though Phila. is an interesting place, too.

Others are right about Atlanta -- both as a place to visit and as a hopping-off point to FL. Lots of flights out of Atlanta to several points in FL.

If you go to Boston at the end of October, and especially if you have older kids in tow, check out the big Halloween bash in Salem (anytime in Oct). Way over the top, and a bit cheesy, but you can also take in the Peabody Essex Museum and the Hawthorne House of Seven Gables! Witch Museum is also decent (not the Witch House which is rather dull!).
Florida is wonderful in October, and most off-season rates are still in effect.

Boston in February, yikes!! It will be frigid. Not that Philly will be warm, but will be better in Philly than Boston as far as the weather is concerned. While Atlanta will have the best weather of the three cities, Philly or Boston have a lot more to offer. My choice would be Philadelphia over either Boston or Atlanta.

www.gophila.com

Check AirTran's website for inexpensive flights from Atlanta to Florida cities - or to Savannah. BEAUTIFUL historic coastal city and head out to Tybee on the beach. Can't tell what the weather will be like then. We've had "short sleeve" Christmases. Played on the beach at Cumberland Island on Dec. 31st one year.
Now, for someone who doesn't love London for more than 3 days, but loves driving the countryside...what about a day or so in Atlanta and then getting out into the North Georgia mountains? Chances of snow? Maybe...but not likely.