The Manix 2 is almost better in every way, better steel, better fit and finish/quality/US made, and I'd even wager it's a better design. It is a bit large(wide and long), so if that's something you wouldn't like just double check the specs with a tape measure.
That doesn't mean the Tenacious is a bad knife by any means. I still have mine and it served me very well, actually what turned me on to Spyderco in the first place. It's just not up the quality of the Manix.
The Manix is one of the the most ergonomic folders I've handled, it feels purpose designed for comfortable and control, both aspects it definitely achieves. If I had to choose one folder for the rest of my life it'd be a Manix.
If you have the opportunity try hitting up a large(ish) hunting shop and get either knives in your hand, it should help with the purchase. And don't forget about the Delica, it's right in the middle between the Tenacious and the Manix in both price and quality.