This campaign slows down the leveling speed of Factions

Nightfall: The Nightfall campaign gives access to the Paragon and Dervish professions. Most importantly, it gives martial empires gold early access to heroes. This campaign slows down the leveling speed of Factions, but still faster than Prophecies.

Access to other campaigns is faster than Prophecies and Factions. This is a good middle ground for the two previous campaigns. If you just want the heroes however, it might instead be better to switch to this campaign from other campaigns to get five of them at level 15.

Skill acquisition is similar to Factions in that players are mostly expected to buy their own skills. The gold rewards are also slightly less than the Factions campaign so a more careful use of gold is necessary.

Elite skills are available in abundance from quite early on. However, unlike in Factions, many non-elite skills are not available from trainers until quite late in the campaign.

You can also purchase skills in Nightfall via Hero skill points. Each character gets a limited number of these (max of 36 in Nightfall), but they can be used to purchase skills from any profession, even if your character has not yet unlocked that profession. The same mechanic is available in Eye of the game.