I tend to split up my on-line business. Most kits I buy from http://www.greatmodels.com/ because they tend to have the lowest prices on the kits themselves. I also get some of my aftermarket stuff from them. Most of my PE sets were bought from http://www.hometown-hobbies.com/home_page because they are a little bit cheaper. However, I've bought kits, aftermarket stuff, bits, and pieces from many other online stores.
Try out a few and see who you like the best. My only advice would be to check out shipping costs. Some times these can run as much as a single kit. However, since most of my online orders were over $100 or $150 dollars, most of my orders were shipped free of charge. If you wait until you have a large order to place, you can save on shipping.
Robert