TL;DR: Use StringBuilder.
The truth: unless you're joining large amounts of long strings, the time it'll take PowerShell to read, parse, and compile the file is going to outweigh any improvements you make in the runtime unless you run the code repeatedly. This is easy to forget about, but try it for yourself. Download and run Strings.ps1
below, and then, run it a second time -- remember, you incur that first run penalty in each new PowerShell session.
Obviously the results vary depending on your strings! The more there are, and the longer they are, the bigger gain you get from using StringBuilder. To sum up: StringBuilder is faster except in very small test cases, but it's not much faster except in extremely large test cases.