Worldwide Marijuana Seeds/Single Seed Centre typically has better prices on single seeds, and you don't actually save any money buying full packs; Attitude usually has better prices on full packs, and they give away more free seeds. I got a free feminized Rocklock (DNA Genetics) from Attitude, and it turned out to be a male. I don't necessarily blame that on them (I would blame DNA) but it certainly didn't do me any good. I have a couple more free DNA Genetics seeds, Sour Cream and Sharksbreath, which I am currently germinating and are hopefully actually female.
I find that shipping with the Attitude ends up costing a lot more, because I feel compelled to get it shipped with an additional product, so if you order in that manner get a lot of seeds to make it worthwhile. Attitude discount code '420' will get you 10% off. I believe the Single Seed Centre discount code is '420MAG'.