It's an age-old quandry. Full songs, excerpts, genres to include, etc. I have tried all and not sure that one works any better than the other.
The last demo I did was specifically tailored to the venue. After soliciting the guy in charge, then being referred to an underling, I burned the tracks with a verse, chorus, verse of about 10 songs, mixing up styles and tempos. The CD label made it clear this was a DEMO specifically for the (names) of club and my contact.
Included in the package was a pic, recent play dates, references and upcoming availabilities.
I specified my rate and offered a one time introductory rate for the first job. More than a month went by. I called several times, could never get the contact person and they never returned calls.
I was just about to write it off as wasted effort, when they called and offered me 4 dates at more than I normally charge. I thought it for about a nano-second before accepting.
Eddie