Depends on what you want.
If you want around 60% more battery life and to double the thickness of your phone and add weight then get the 3500mah.
If you want to add around 5-10% life (at best) to your phone without adding any weight/size then get the Anker.
I have an extended 3500mah and the Anker. I run the Anker daily and it is just a good fresh battery that gives a hair more time on the battery. I keep the 3500mah around in case I need to travel or something. The 3500mah is just not really practical unless you are stuck using your phone all day and need 8 hours of screen time before you can get to a charger.
Just remember that with either one you want to do a fresh charge on first install. Clear your battery stats in the recovery menu, charge it to 100% with the phone on, then turn the phone off and let it sit on the charger for a while, then turn the phone on and leave it on the charger a bit longer. That ensures it completely tops off and the system learns where the 100% mark is at.
