Replacing battery: Is there a difference with "extended" batteries?

Hello! The answer to my question is probably online somewhere, but I can't find it through the mess of irrelevant information. The story is below, but if you want to skip to the question it's at the bottom.

I'm replacing the battery in my iPod Touch 4th Generation, and I've been looking at batteries online. They're scarily cheap, and I'm wondering if they are poor quality (if anyone knows that'd be great help!). For example, on eBay, here's your run-of-the-mill battery listing:

http://www.ebay.com/itm/Original-Li-ion-...

It says it's an "original" battery, which gives me a bit more confidence, but I'm still wary. Then I started wondering if there is an extended battery that will push me past the expected 7 hours-on-one-charge. So I found this one:

http://www.powerbookmedic.com/iPod-Touch...

It doesn't list the mAh, unfortunately. The "extended" battery is three to four times the cost of the "original" battery, but if it really improves the battery life I'd be willing to spend $20 for it.

But from what you guys know about batteries, is it possible to have a battery of the same type (Li-ion) and size with a greater capacity for storage and discharge than the OEM version?