Can somebody please explain me the difference of T1 Reference vs T1 Sandwich? Thnx
The difference lies in the internal layout. The external layout of the case and structure stay generally the same (with the only difference being a different top panel and different rear I/O panels at the back)
Think of the T1 Reference (abbreviated as T1-R) as having the internal layout of an Ncase M1. This means the GPU plugs directly into the motherboard, with no riser cable.
Think of the T1 Sandwich (abbreviated as T1-S) as having the internal layout of the original T1, or the DanCase A4. This means the GPU sits up against the back of the motherboard, connected with a riser cable.
The T1-R is mainly for people who have FE GPUs, or GPUs that are 2.25 slots or less and no taller than 112mm, while also being no longer than 320mm. It allows for tall air coolers like the Noctua C14s to be used.
The T1-S is the best for most people. It has great versatility, and allows you to install bigger GPUs (up to 3 slots thick, and 140mm tall) at the cost of CPU cooler clearance. This means that 240mm AIOs are the general go-to for this configuration, as the pump block for the CPU is not very tall, and you get good cooling from the radiator.
You can look at the specs for each layout
here.