I own both and rarely carry either because I have a lot of other knives that are better than both of them.
Both are totally functional knives. I like the Tenacious better than the Rat 1 because it just suits my particular desires/biases better. For some reason I view the Rat1 as more of a camp type knife than one for EDC use but again that's simply based on my own particular perspectives.
Frankly, I don't think you can go "wrong" using/carrying either knife for normal use but, if you're you've got a limited budget, the Rat1 is obviously the better buy because it's cheaper.
If I had to let one go, it would be the Rat1 because I really don't need/use it. I'd keep the Tenacious (again because it suits me better) and also because I consider it one of a 3 knife Spyderco family subset which includes the Resilience, Tenacious and Persistence. I'd add the Ambitious to the subset too but that's just too small of a knife for me to bother collecting.