Any battery that is the same size will not have a higher capacity, no matter what the seller tells you.
The higher capacity batteries are bigger and uglier, much uglier. Just buy another OEM battery and carry it around with you. That way you don't turn your phone into some hideous fat mutant and you double your battery life for just a few
If you register your phone with Samsung you'll get a 50% off coupon for accessories, good for 30 days from the date of registration. You can use it for a second battery & the coupon brings the cost down to 20 bucks, or a battery with a separate charger for $25. I found a kit on Amazon that contained two aftermarket batteries and a separate charger for $25 before I learned of the Sammy deal. It is listed as an Onite 2x 2600 mAh Extended Li-ion Battery for Samsung Galaxy S4 with Charger. The batteries seem fine as I used one of the batteries yesterday and had 55% left after 24 hours. I also like the idea of having a separate charger that operates independent of the phone.