SW OK has the Whichita Mtns, a few trails. Eastern OK is kind of like Arkansas, lots of hills, forested, and some bigger lake/river/swamp complexes, plenty of ticks. Otherwise flat and low, some Not as humid as most of TX, more tornados.
Hard to pick which is worse. TX still gets tornados, no big Mtns except in far west near El Paso painfully flat elsewhere. Swampy to the east. It is big, I've been all over it, but it's mainly a lot of nothing interesting. Also, for all the "wide open" stuff people talk about, there are very few, if any, areas that are not privately owned and very few spaces you can just "enjoy" as your own.