I'm gonna be honest here, I don't think there is very much competition involved, hear me out.
To start with, the Manix 2 is around 120$ a max of 160$ more or less depending on where u live. While the tenacious sits around a 50$ and a max of about 70$. This means theirs obviously something in the Manix that makes it more expensive. (Probably the CBBL lock)
Second (probably the most important), the Blade on the Manix 2 is made of the ever so common S30V (I personally love S30V good edge retention, soft enough for sharpening and chip prevention), while the Tenacious houses a blade made of 8Cr13MoV. Now I'm gonna say this right now, I don't have much experience with 8Cr13MoV, but from what I have read S30V has higher edge retention, holds an edge longer, but is harder to sharpen and takes longer. The dimensions on the 2 are very similar (I personally have the Manix 2 XL so it is larger) but the blade structure of the Manix 2 compared to the Tenacious yields a stronger tip.
A few small things to take in to consideration.
The Tenacious is tip up and tip down carry, while the Manix 2 is strictly tip up.
The Manix 2 blade is a small increment thicker 0.118" in the Tenacious, and 0.125" in the Manix 2.
The Tenacious is lighter due to milled out handles. As implied the Manix 2 does not.
What I think it comes to is the Manix 2 will go threw more beating and abuse than the Tenacious. The reason I say this, is because the CBBL lock is considered Spyderco's heaviest duty lock. The blade is a bit thicker, the liners aren't milled. I dislike liner locks, because i feel they take away from the integrity of the handle having the G10 grip and steel liner split. (G10 can crack easier on the liner lock side of the handle.)
Conclusion comes to, if you're scared a Tenacious might break with the work your using it for, get a Manix 2.