I always buy extra batteries for my phones. Note 1, Note 2, S4, and now the S5.
I have yet to see a battery that was the same size as OEM and had higher capacity. Anker had one that I tried that had I think 100ma higher capacity, but when I placed the battery in my phone it was a tighter fit. So really in my books it was a bigger battery.
I am not saying its not possible to get higher capacity than OEM in a slim battery, but I have not come across a slim battery that met lofty claims yet. For this reason, I now tend to stick to an OEM battery as I can trust that it will operate in a predictable manner. Add in the waterproofing seal on the current S5, I would try my best to stick with a battery that maintains OEM dimensions.
Current: Samsung Galaxy S5 (SM-G900W8, NCG baseband, Bell Mobility) , Sony Tablet Z 32GB
Past: Galaxy S4 (i337m Bell Mobility), iPhone 4, iPhone 4s, Galaxy Note, Galaxy Note 2 i317M, Toshiba Thrive, Acer a500, Asus Transformer TF101, Asus Transformer TF 201, Asus Transformer Infinity, Acer a700