Nightfall.
1) Professions are better IMO. Paragons make great characters for party support with their chants, shouts, etc. Dervishes are awesome melee characters especially with their enchantments, form elites, and scythe skills.
2) Better storyline. The storyline in Factions felt rather empty and the whole thing felt rather tiring and annoying.
3) Heroes – Only in Nightfall. You get up to 3 henchmen to use in your party at any time in any campaign. And these henchmen you can customize their armor, weapons, skills, and even their approach (aggressive, guard, or passive). And you can even dictate their skill use if you choose.
4) Factions is primarily a PvP chapter. Lacking in PvE. If you need a green item from the campaign, let me know.