There's no big consensus on which way is best. Some people go with alts, some go with everything on one character.

Pros of a single character:
-No need to redo content. Once something's unlocked it's unlocked.
-Easy inventory management. You can't send items between your own characters, so it's tricky to pass things back and forth (FC chest or trusted friends or extra accounts).
-No need to repeat classes. Every job takes 3 classes (two to unlock, one more for cross-class skills). On a single character, you won't need to level your secondary and tertiary classes over and over.
-No need to log in and out, no need to re-establish friends and/or remind everyone who's who.

Pros of multiple characters:
-You can see the other starting cities' story lines (this is only a thing until level 15ish).
-Different races.
-Extra shots at daily/weekly lockouts.
-More overall inventory space.
-Potentially less cluttered inventories (carrying around fewer sets of armor, not saving crafting mats on a character that doesn't craft, etc.).
-Potential ability to hide from some people.

Most of that assumes you'd put your alts on the same server, of course.