Best deal on ATT's network is Straight Talk. Unlimited talk, text and "4g" data for $45 Usd per month.
The 4g data is HSPA+ not LTE but still quite fast. US cell carriers have a different definition of unlimited than the rest of the English speaking world.
Many customers on the straight talk forum claim upto 2gb is safe but others claim they use much more data. Straight talk will throttle you if you use too much data which they won't define.
Rather than pay for a separate data plan, I would tether the Ipad too the Iphone.