You totally should. TBH, Dark Below isn't great. The Raid is nice, but everything else, -.- no.
House of Wolves is definitely worth your money. The exotics are all great (except Eternal Warrior, it's ugly and bad) you get Prison of Elders, extra (easier) bounties, Trials of Osiris, a new social space + cryptarch (who imo is worse than Rahool).
With HoW you basically get 2 sets of raid gear (Osiris and PoE, though Osiris is PvP). Instead of just one Exotic Raid Weapon (Like Necrochasm and Vex Mythoclast) you can get 3 from PoE, Dreg's Promise, Queenbreaker's Bow, and Lord of Wolves.
Once you get past Lvl 20, it gets a lot more addicting tho tbh. I also recommend grabbing a Faction Class Item once you hit Level 20. Every rank up you get towards a faction (After level 3), you get a package that can contain exclusive Shaders, a Ship, and a Sparrow.
Anyway, Reset happens soon. I bet the Nightfall will be Aksor and the Heroic will be Phogoth. IIRC the PoE will be- 32: Gulrot, 34: Valus Trau'ug, 35: Skolas. Also the Armor piece will be the Helmet, if I'm not mistaken.