Isn't Earl the "good" title to have? That is, the holder tends to be of an older family, "duke" being somewhat arriviste. That's been my impression, however, incorrect.
That's too much of a generalisation I'm afraid. The oldest existing (non-royal) dukedom is Norfolk, created in 1483. The newest (non-royal again) dukedom is Westminster created in 1874. Most dukes held other titles before they were made dukes. For instance, the present Duke of Norfolk also holds the barony of Beaumont which was created in 1309.
The oldest extant earldom is the Scottish title of Mar created in c1115 and the newest is Stockton created in 1984 for the former Prime Minister Harold Macmillan. Most are 18th and 19th century creations, so not much different to the dukedoms in that respect.
As asadoyayunta says, dukes are two degrees higher in the peerage than earls and generally very much grander.
It is incorrect. Duke is a superior title to Earl, the highest non-royal ranking. As said above the elder sons of Dukes will often take one of the father's junior Earldom titles until he succeeds. For example, the Duke of Norfolk's eldest son is the Earl of Arundel.
The eldest sons of dukes generally take their father's most senior non-ducal title. That's not always an earldom. For example, the eldest son of the Duke of Argyll is the Marquess of Lorne, the eldest son of the Duke of Manchester is Viscount Mandeville and the Duke of Somerset's eldest son is merely Lord (ie Baron) Seymour. Despite that, Lord Seymour actually out-ranks the Marquess of Lorne and Viscount Mandeville, because precedence depends on the date of creation of the Dukedom itself and the Somerset title was created before Argyll or Manchester.



















