Oak without question. You're right regarding the pine being soft. In addition oak has more grain. Unfortunately there isn't a product out there that will hold up to the CO environment. You & I will be refinishing our California beds every two years. Stripping & refinishing wood slats isn't really that big of a deal. Personally I wouldn't buy the wood kit but rather rip the slats myself for 1/3 the cost.
1970 FJ40- TBA