Overall it is cheaper to go with the iPad 3G, but if you are like most of us you have a smartphone that already has a data plan and can be tethered. This would be your best and most cost effective option.
Try the whistle phone app. That allows you to make telephone calls off the iPad. I tired it and it seems to work.
Thanks for the comments about Skype and Whistle! I'm usually at a place with good wifi. (I'm semi- retired and don't choose to pay for a smart phone.)