Kettlebells are of two kinds: competition and not competition.
Competition kettlebells are the traditional shape, with square handles, and meant for one handed use. Being the same dimensions, they are also held the same regardless of weight.
However, for many purposes now advocated, where the overhead lifting is de-emphasized and two handed lifting is common, the newer version is sometimes better suited, although, for overhead lifting, the varying sizes can cause issues, especially if the size makes it impossible to hold properly.
For competition bells, I cannot give advice.
For non-competition bells, I say get whatever is the cheapest which works. CAP Barbell makes good cast iron kettlebells and they have free shipping on Amazon.
Christians Fitness Factory has good prices too:
http://www.christiansfitnessfactory.com/
StrongFirst sells Perform Better kettlebells as far as I know:
http://www.performbetter.com/webapp/wcs/stores/servlet/Category4_10151_10751_1000420_-1_1000156_1000156_image_0
They have competition bells too.
My first ones were the old DragonDoor ones (before they apparently changed the supplier) and they were the only real source back then that I knew of. Now, everybody is selling them and they are a commodity, so do not look for hidden quality in cast iron. If the handles do not have burs, and they stay in one piece, then they are good enough. They are still expensive (in contrast to the Soviet reasoning...cheap and easy to make).