The Marshall. Hands down. For tone and value.
No kidding.
This is a 'trick question' right?
The Plexi is THE sound of rock and reminds me of everything that I grew up listening to and why picked up a guitar 30+ years ago.
This is a freakin trick question! Both are awesome, I suppose with the Komet you get modern engineering and quality and a builder who could service, while the '68 you run the risk of the well mojo'd parts going south and you need to find a competent tech to service. Either one kicks a$$ tone wise, so it's hard to go wrong either way in that department.