The 3GS isn't more powerful, the reason it doesn't have the battery percent is because if it did it would be inaccurate. The reason for this is that Apple didn't include the hardware to accurately determine the battery.
Are talking about the numeral percentage next to the battery symbol like the iPhone? I wondered the same thing, I figured it was hardware except when you jailbreak you can turn it on just like that an it's perfectly accurate. I've had the percentage for a year. I don't know why apple doesn't just allow it