The distinct advantages of PETG are that it’s much easier to work with for both beginners and experienced users looking to do unconventional bends and less expensive than Acrylic while maintaining relatively high clarity. Once the system is filled, many cannot tell the difference between them unless looking down the length of the tube. It can even be cut with standard tubing cutters. The downsides are that it is not as tolerant of high coolant temperatures as Acrylic or glass and isn’t as chemically compatible as them, either.
The advantage of Acrylic is the blend of higher clarity and greater chemical resistance than PETG, but lower cost and substantially greater ease of use than Glass. It can also be worked with high grit sandpaper to produce a semi transparent satin finish. The notable downsides are that it is more expensive than PETG, cannot be cut with tubing cutters, and isn’t as scratch and heat resistant as glass, nor can it be cleaned as easily.
Glass has unparalleled clarity, heat resistance, chemical resistance, and scratch resistance among the typical choices for transparent tubing. It can be scrubbed inside and out with soap and brushes without ruining the appeal and leaving scratches on the inside. It’s used in labs for a reason. The downside is that it is substantially more difficult to work with than either of the other two options, requiring practice with specialized tools to pull off well. Because of this, very few choose to use glass tubing over Acrylic.