AN msi is an installer, uninstaller and program repairer, it performs all those functions. The same file that installs the program also uninstalls it and if the programs detects missing files or corrupt settings the msi repairs it. When you install a program with an msi, a copy of it or a near copy is also installed somewhere so that the program is always able to be repaired without you having to really do anything.

C:Windows Installer is one of the folders where back-up msi are kept. If you have a choice choose msi.