If you had no issues with your own production Emersons, than why would you worry so much about what other people say about Emerson's "poor" build quality?
But back on topic about Emerson vs ZT framelocks. Here are the main differences, Emersons are noticeably lighter, newer production versions run on ball bearings, and the blade stock is a little thinner which could mean better cutting performance in some situations. The smaller blade stock also makes the Emersons slimmer in handle design to make them a tiny bit more comfortable in the pocket. I know some people don't like the bearings, but I personally like them.
On the other hand, the ZT 0620/0630 uses newer steels, runs pretty smoothly with Phosphor Bronze washers, is drilled and tapped for left/right carry, and has a more rigid body than the Emerson framelocks.
So, which ever set of benefits appeals to you more should help steer you towards an Emerson or ZT. A lot of people on BF put a lot of weight on blade steel, which makes the ZTs more attractive. If the HD7 tanto is not for you, than the bigger Iron Dragon might be a better comparison for the ZT0630