Gus always uses Microsoft-related products while Shawn always uses Apple-related products. This includes their phones in later seasons where Gus can be seen using a Windows phone (e.g. s7e6 "Cirque Du Soul"), and it's well-known that Shawn uses an iPhone. Gus always uses a PC, and Shawn uses a Mac. Except for in some episodes where an Alienware laptop can be seen on Shawn's desk. This can clearly be seen in the episode "Forget Me Not".