There is a right time for both.
Pipe systems ought to be the standard. If your systems are set up properly and are properly modeled, every thing in that piping system will be assigned automatically. If you set up filters to provide graphic overrides based on the systems, you can very clearly see where your system is not modeled properly. There are instances where modeling it properly can seem counter intuitive, for instance in bypass lines and recirc lines. When you have one system bypassing to another system without a piece of equipment in there to separate the systems, Revit gets a bit confused, but there are easy work arounds.
The problem with pipe systems that I frequently run into is that they don't carry over well in multi-phase projects. If a pipe is being demolished in any future phase, the pipe will automatically be placed on an undefined system. If in the current phase you want to tag the pipe system or use a graphic override that reads the pipe system, you won't be able to. You will need a separate parameter that requires user input...the pipe type parameter works great for this.
If you only use the pipe type parameter and completely forego assigning systems, users will be able to model things improperly much easier. For instance, you could have a pipe on your hot water system be assigned the pipe type Chilled Water which could completely mess up your drawings and tags and result in a lot of problems down the road.
