One has a big parade in several cities (St. Patrick's Day); the other does not have a big parade in most cities.
Depends the city you live in. Places that have fairly large Hispanic populations also tend to have fairly large Cinquo de Mayo celebrations. This is true not only in the Southwest where Mexican-Americans are often a majority but in major cities on the East Coast as well. I believe there's a fairly substantial celebration in Spanish Harlem in NYC, for example. (OK, it's a not a parade on 5th Avenue but, then, most Hispanics wouldn't attend a parade on 5th Ave.) There's a major celebration along Blue Hill Ave. in Boston's Dorchester section. (Again, this ain't St. Paddy's in Southie, but there are damn few Hispanics living in Southie.)