Now that the iPhone SE has been announced, the iPhone 5 and 5s are the new low devices on the totem pole. The iPhone 5 may not be left out as much as the iPhone 4s when it comes ot new features, but its older hardware is beginning to limit what it can do. For example, the iPhone 5 will not support Night Shift mode, and it does not have the hardware to support most of Apple's Health app features. The iPhone 5 does support CarPlay, which means it can take advantage of the new Apple Music integration found in iOS 9.3. It will also support the new features found in the Notes app.
So should you upgrade to iOS 9.3? Unlike the 4s, the iPhone 5 can take advantage of a few of the new features found in iOS 9.3. Not to mention, this is probably the last big release that the iPhone 5 will really support. You may be able to upgrade to iOS 10 when it's released, but iPhone 4s owners will be able to tell you it probably won't be worth it. So yes, take advantage of Apple's latest update while you still can. It will give your iPhone 5 somea longer lifespan and cadd ouple of new features.