The two knives both have the same blade steel, so that's a moot point. General strength is probably comparable (insofar as in both cases, the blade is likely to fail far before handle or lock strength becomes an issue) so it's really a cosmetic and ergonomic distinction.
I had the same decision to make awhile back; I went for the Spec Bump. I used to be skeptical about the lock, but now I see it basically as an Axis lock contained in the blade instead of in the handle. That aside, the ZT, though it was the lighter of the two, felt really uncomfortable in my hand. The Spec Bump, meanwhile, fits me like a glove. Also to me, the Bump looks meaner, and yet simultaneously more elegant. The ZT is a bit of a brute by contrast.