If you will constantly use your iPad via the cellular network (either with built-in 3G or tethered), then buying a WiFi-only iPad and tethering to your iPhone 4 is the less expensive way to go.
If, however, you will only occasionally need to use your iPad on the cellular network, it is less expensive to but the 3G-capable iPad and buy 30-days worth of data as you need it.
Also, you lose the GPS feature when you buy a WiFi-only iPad. So, if you have thoughts of using your iPad as some sort of navigational aid over 3G (turn by turn directions, etc.), you will want the 3G-capable version.
Lastly, keep in mind that when you are tethering you are using up battery life on BOTH devices.
For me, it seems worth the extra $130 just to get the ability to use built-in 3G data and the GPS. Just because I buy a 3G-capable iPad doesn't mean I have to use its built-in 3G. I could still tether it over WiFi and only turn on a 30-day supply of 3G data in the iPad if I feel I absolutely needed it. And this is one of the reasons I am considering getting an iPad 2 that is on Verizon. My iPhone is on AT&T and I have no intention of switching as my AT&T service is outstanding throughout my area. But when I travel, AT&T can get spotty. Having an iPad that can use Verizon instead would be helpful when traveling.
Mark