Is Nam Nao worth a day or 2 exploring and how does it compare to the other park?

On my visit to NN, i walked around a loop trail near the highway for an hour or so. It was gently rolling terrain as I recall. At PR you will face a steep climb of about 3 hrs. to reach the plateau. There are porters available, though I have no idea what they cost. Once you are on the plateau, the terrain is not especially challenging.. If you need a staging point, use Chumphae, Khon Kaen, rather than Meuang Loei. Chumphae is closer to the park, and closer to all points south.

Much worth seeing or same same?

Nam Nao is in the hills, but not especially interesting, vegetation wise. Phu Reua is higher, so a little more alpine. Thai's go there to take pictures of frost on the vegetation during the cool season.