If you want to know what I'd do, I'd keep it and buy Skyward Sword. I'm also 28 and don't see any reason why I should stop enjoying videogames. It ceased long ago to be an activity that only children participated in. It's just another form of electronic entertainment just like music, television shows, and movies are. Nothing unusual about a male in his late 20's enjoying a good console game.
As long as it doesn't interfere with your real-life responsibilities, I don't get why you'd feel guity enjoying a videogame. I imagine the average age of a gamer on the Xbox 360 and Playstation 3 is right about where we are. And if you're looking at videogaming as broad as possible, we're both well below the average age of a videogame player.
I say enjoy it and get over this sense that you shouldn't enjoy this hobby because you're in your late 20's. You're going to get more enjoyment out of playing through Skyward Sword on it than you would selling it and getting a small portion back of what you paid for it (And there's plenty of other Zelda action to be had on it via backwards compatibility with the GameCube and the Wii's Virtual Console).
A Wii with Twilight Princess isn't going to fetch you much off Ebay or Craigslist these days. Unless you absolutely have zero interest in enjoying it ever again (And clearly, you do have some interest or you wouldn't of posted this), I think it makes more sense to keep it and try to enjoy it.
Here's some other Zelda action that is available to you on the Wii, if you're interested and it helps sway your decision (Every console Zelda title is here, other than those non Nintendo developed or published CD-I atrocities from the sillborn SuperNes cd expansion deal with Philips). Only the handheld games are unavailable (Too bad, Link's Awakening DX and The Minish Cap in particular would've made for fine downloadable releases on the Wii).
Legend of Zelda - NES (Virtual Console)
Zelda II - NES (Virtual Console)
A Link to the Past - SuperNes (Virtual Console)
Ocarina of Time - N64 (Virtual Console)
Majora's Mask - N64 (Virtual Console)
Link's Crossbow Training (Wii)
Wind Waker (GCN)
Four Swords (GCN)
Collector's Edition - Contains both of the NES games and both of the N64 games (GCN)
Ocarina of Time Master Quest - Includes Ocarina of Time and a new 2nd quest (GCN)