Is the 3G network used by the iPad the same as the 4G network used by the iPhone?
iPhone 4S supports HSPA+, while iPad 2 doesn't.

HSPA+ is technically not 4G, but is being advertised as 4G by networks like AT&amp;T and T-Mobile in US. Pre-iOS 5.1, Apple wasn't calling HSPA+ a 4G network, but they have started calling it 4G now to match AT&amp;T's other "4G" phones that are using the same signals and protocols as the iPhone.

If you are in HSPA+ supported area, you should hopefully get better speeds on iPhone 4S.