The 3G chromebook is without doubt the most convenient choice, but if you already have a smartphone with a WiFi tether app then you can save a bit on the purchase price and avoid the need for a separate mobile data contract.
You might also upgrade your smartphone to 4g within the chromebook's life, so this cheapskate approach is arguably more futureproof.
The downside of this penny pinching is that you will be draining the smartphone battery. I invested in an extended battery for my HTC Desire to help with this.
It really depends a lot on where you'd use it, I would think. If you are mostly using it at home around a wifi connection - or at restaurants or hotels that offer wifi - then 3G would be pointless. If you want to use it on the road, then you might want the 3G.