This might be just as valid on english.stackexchange.com - great place to discuss the nuances of English words and grammar.
I think abuse is more broad. It covers more, but some timid/unexperienced users might not know exactly what qualifies as abuse and avoid using it. They might think spam isn't considered abuse so they wouldn't report it.
Spam is pretty specific to emails and other forms of unsolicited communications. You might not report a DDoS attack to a spam list, but Grandpa Dirt who just got his first computer last week surely has a general idea of what spam is from watching the TV (news, etc) over the last 15+ years.
I think you might be splitting hairs with this one. But if it's important, then I would say it depends on your community and what you're expecting exactly. If it's specific to unsolicited comms I would say stick with "Spam". "Abuse" is a pretty known term in the techy community. Most ISPs and domain registrars have had abuse emails since the mid 90s (or earlier).