i would definitely say the kickboxing class, since its a class you ll be paying for you ll pretty much HAVE to go, and a professional will be running the class so he/she will know exactly how to help you lose weight =]
It depends how fast and where you were riding your bike, but i would suggest doing both that way your excercising different muscle groups and also bike riding can be very relaxing so you d probably stick to it longer... and also having a day off or just riding slowly so your body can recover.Variety in your excercise is just as good as variety in your diet!yeah the treadmil sounds like a good idea aswell...and bike riding also depends where you live, i live in a small town with heaps of off town roads and so i can ride fast without really having to worry about traffic, it is a great workout =), but if your in the city it might be hard...