If you use a online booking service like 12Go Asia, you can usually choose seats. And sometimes there are seats with amazing legroom for this short trip.
Tuktuks will very willingly take you to the bus station at Nong Khai, where you will find a bus to Udon Thani for sure.
But there are probably taxis too. I did not really pay attention to them though. I was headed for my bus and got jumped by eager tuktuk drivers that may have overpriced my journey just a smidge.