I am assuming your iPad has both Wi-fi and cellular capabilities. And when you acquired it, you never installed a SIM card in your iPad, using it only on Wi-Fi- at least until now?
The choice of a cellular provider depends on where you are going in the US and the coverage. I have used both ATT and Verizon and both have good US coverage in major cities. So for me, either one is fine. Which company do you hate the least?
I have two iPads, one of them is Wi-Fi only, and when there is no wi-fi, I use my iPhone's cellular plan to tether (connect) them and that has also worked. And that's the scenario I have used Overseas in the past. ( i.e. my iPhone had a European SIM card with a generous data plan and I just tethered my iPad to the iPhone when there was no Wi-Fi.)
In the US, I use my other iPad (with both the Wi-Fi and Cellular data plan) on a daily basis with either ATT or Verizon, and they work just fine in major metropolitan areas.