I've got three months before a holiday...

What are your goals? Just to lose weight or to firm things up?
If the former then just cut calories and do cardio, preferably in the form of HIIT work of some sort. I recommend this article:
http://www.nerdfitness.com/blog/2009/01/27/kick-your-*kitten*-and-kickstart-your-metabolism-in-20-minutes/
You will get more burn from a shorter, more intense (soul crushingly intense in some cases) workout than from an hour long fitness class.

If the latter then resistance training with weights is your friend and I recommend this article: http://www.nerdfitness.com/blog/2011/07/21/meet-staci-your-new-powerlifting-super-hero/

If both then do alternate days.

I would still work both of them together. You will see more changes in your body by lifting heavy things than by cardio and I think you would be more pleased with the results. Besides, building muscle raises your resting metabolism, so you are actually burning more calories while sleeping and sitting than you are now. Maybe focus on the HIIT 3 or 4 days a week and work in 2 days of strength training (Which can just be bodyweight squats, push ups, etc... at first) and then, as you near your goal weight, reverse and have more strength than cardio. If you are dedicated, in 3 months you can truly be a whole new you! You don't have to post them online, but make sure you take a before and after picture!