Nothing wrong with the Ontario RAT knives, the ESEE is just a slightly modified/improved model with better attention to finish & fit.
slightly longer handle, scales individually fit to the knife, more consistent heat treat due to knife being made in smaller batches, hand sharpened, molded sheath designed for that specific knife, and the warranty -- which is there so that if you're ever in a situation where you NEED it, you're not afraid to use it for fear of breaking the blade.