> So I guess my question is: what do you see as the reasons why you'd pick a particular school of micro control flow as a microcode engine implementer?

For a comprehensive answer, a good vintage introductory digital design textbook is Ward and Halstead's 1989 Computation Structures, from the "peak CISC" era! [1]

There, the second (vertical) type is often used for highly complex instructions/fancy addressing modes, that you might want to implement with some sort of procedure abstraction, loops, working memory, etc. A "luxury" vertical microcode engine would have facilities like "microprocedure calls", a micro-stack and workspace RAM, a micro-ALU, dispatch table micro-instructions. The authors use the suggestive term "interpretive microcode".

String instructions come to mind as a complex example; non-register machine architectures (stack machines); tagged data architectures that have instruction-level polymorphism (e.g. Lisp machines).

The culminating project of Ward and Halstead is an elaborate two-level microcode system (vertical on horizontal/second on first). I think the first Motorola 68k had this architecture -- here is the patent. [2]

It's genuinely a fun read. The "write an micro-interpreter for your CISC ISA" approach is hopelessly out of date now that we need pervasive microarchitectural parallelism, and have HDLs.

[1] https://www.amazon.com/Computation-Structures-Optical-Electr...

[2] https://patents.google.com/patent/EP0011412A1/en?inventor=Ha...

Here’s a great book explaining microprogramming from ground up: https://www.amazon.com/Computation-Structures-Optical-Electr...

Easy to find a free pdf