Actually - the "music only" CD-Rs are nothing more than a cash-grab..... the difference is that music CD-Rs have the copybit "pre-burned" to on. Most consumer-oriented digital equipment observes the copybit setting (pro equipment does not) and prevents digital copies being made from CD-Rs with the copy-bit set.... but the cash grab came into play because manufacturer's created machines that REQUIRE you to use these "music only" CD-Rs and charging higher prices for the media (while at the same time, leading consumers to beleive that the music CD-Rs are "better-sounding"...)
There is no difference in the physical media itself, but machines can be made to accept only CD-Rs with the copy-bit set (kind of the opposite intent of the use of the copy-bit in the first place, but anyways...)