Now there is a compelling reason to step up from a WiFi-only iPad. With 4G LTE, your transfer speeds can reach as high as 73Mbps, far better than 3G. 3G will still work, even around the world.
Finally your iPad can outpace your iPhone. This makes it more practical to get a 4G iPad than tether to a slower iPhone, and share that data plan. In fact, your iPad can now be a personal hotspot, to share its 4G LTE goodness with other devices, if your carrier supports that.
Make sure your carrier has decent 4G LTE coverage in your area. I might use Verizon for my new iPad while I still use AT&T for my iPhone 4, to give me the broadest coverage.