With the battery, it doesnt add too much weight. But me personally, i prefer the phone to feel kind of heavy. I dont like how some of the Galaxy S phones feel so light, they feel fake or like they will break after some usage.
Seido has just been around longer and have really made a name for themselves in turn of the products and customer service. And plus everywhere where i saw the other batteris being sold, they were also on sale for some reason, as does seideos batteries go on sale as well