I've owned both and have sold both. Neither one suited me at all. The Endura I had was an ATS-55 Endura 3 and the Tenacious was, well, a Tenacious.
The Endura had a plastic handle (which isn't really to my liking), was very wobbly, and was just too large for my uses. Additionally, it didn't fit my hands at all. Not a bad knife, aside from the blade play, but really not suited to me.
The Tenacious had a bunch of really effing sharp spots on the handle which dug into my hands and made for some pretty sweet blisters, blade steel that literally could not do a day's work without chipping all to hell, and it had the unfortunate habit of opening in my pocket. I worked in a warehouse at the time I owned it and the Tenacious could go no more than a few hours into each shift before it got too dull to be of any service...90% of what it was being used for came down to opening packaging and cutting cardboard boxes. By comparison, I could use my Kershaw Cyclone for three or four days without it getting that dull. I had to sharpen that Tenacious more times in the short time I owned it than I have most any other knife, because the steel just couldn't stand up to any actual use. This is why I laugh whenever someone tries to convince me that 8cr13MoV is 'decent steel' - my experience with it was quite different.
So, I've had both and I don't like either one. If I had to go with one, I'd go with the Endura - it's not really a bad knife, but it's not my cup of tea. The Tenacious, on the other hand, is just not a good knife at all (imho).