Also consider how much time and money it takes *you* to install the systems in the first place. Especially if the PCs need to run an office suite, music, video and photo software.
What I've personally done is:
a) boot up Ubuntu off USB stick
b) wipe the whole hard drive using "dd if=/dev/zero of=/dev/sda bs=10M" (10 secs your time, 20-30 mins elapsed [typically ~2GB/minute])
c) Install Ubuntu (1 min your time, 10 mins elapsed when using USB)
d) Install updates (1 min your time, 30 mins elapsed if off internet, 10 mins if off USB).
If you are installing lots of machines, consider updating the USB with all updates. If you have older machines, consider Lubuntu instead. The OEM install is nice if you want the recipient to create the initial account.
If you need to use Windows, the same principles apply. Use a USB drive to install it, it's faster than CD. Download all pieces of software and updates, also to USB. But you will need to run rather more installers than with Ubuntu. How much is your time worth?