DVD media might be cheap but a little bit of dust etc on the DVD itself or the reader and you've got a Coaster. I know you can get really cheap external DVD devices for those machines that don't have a DVD reader any more - but the only use I have for that now is for Ripping DVD's to HDD so I can stream them with my media streamer when I want to watch them (also works for Blu Ray too at full 1080p HD). I haven't purchased a physical CD for YEARS now either !!! - the sort of music I like I can get quite easily in FLAC format (uncompressed) without problem. I ripped my old CD collection a while back too.
USB sticks can fail but it's much rarer. Good move IMO for Ms -- Unless your computer is REALLY old you will be able to boot from a USB device. Even Protected boot machines can do this -- just disable the protected boot in the BIOS -- however for Windows installation Ms might make the USB device bootable even with protected boot enabled.