Oke the 304 up to Korat, great road almost everywhere 4 lanes, but keep to the speed limit, as there can be a lot of Police Checkpoints on that road (especially when you hit the newer section after Kabin and before Wang Nam Khiao)
The 226 is almost great (97%) as it's now almost everywhere (from Korat to Buriram) 4 lanes. We travel it regularly because I am going to Korat for Hospital check-ups. Used to take us 2,5+ hours last time it was less then 2 hours.
If coming from Korat City, I would now prefer taking the 226 instead of heading back to the 24! (2 years ago, it would be the other way around)