Hi Steve and welcome!

Short answer the Zkit1 is the circuit board kit. Decware supplies the board and schematic/parts lists are avaiable online. It has solid state diodes. A hand made chassis would be needed.

The 5u4 schematic is from an old kit Decware offered years ago. It was a point to point assembly with no circuit board. The kit is no longer offered by Decware but with some research there is enough info online to build a clone using a hand made chassis.

As far as sound differences, the diodes work well and seem not to effect sound quailty. The 5u4 can create a bigger sound stage from most reports and my 20 minute window of listening to it.

Zkit1 is easier to complete. Point to point will take more leg work and a bit of a learning curve to understand how the circuits work. There is also a series of YouTube videos on the assembly process of the discontinued point to point kit.

John