I couldn't vote in the poll, because my answer to the question simply isn't there - my answer is BOTH.
Common decks that look cool are great.
Cool common decks that come in a wide variety are awesome.
Limited run decks often have some of the best quality and design work.
But I always follow a few rules of thumb when I buy decks.
First: do I REALLY want this in my collection? Is it that cool or unique? If it's just a minor tweak on basic Bicycles, then unless there's something else cool about them, I won't bother. If it's a common deck but a hard-to-find variant, like Tally-Ho in Bridge-Sized or Miniatures, my interest perks up a bit.
Second: do I have the money to buy this deck? This should really be first, but hey, I'm human...
Third: just how expensive is it? Is it worth the cost? A deck at $20 a pack gives me pause. A deck at $30 a pack likely won't end up in my collection unless I can trade for it. (This is the biggest reason I like getting something rare like Black Crowns or Gold Arcanes free or at original retail - they make great bargaining chips!)
Fourth: if I'm hung up on any of the previous rules, I take a deep breath and remember this very important maxim: "I WILL NOT OWN EVERY SINGLE RARE DECK, NOR EVERY SINGLE COMMON DECK, THAT I WOULD LIKE TO OWN!" If I did, my girlfriend would kick me out, and she's worth more than any deck/brick/gross/entire print run of playing cards. Housing such a collection would be ridiculous as well - having that many cards doesn't give you a chance to appreciate what you have.
So, I'm a collector, I'm a magician/performer, and I play cards - mostly solitaire, but other games when I can talk my GF and her son into it. I like actually using my cards as well as looking at them. A sealed box is worth more, but not quite as enjoyable for me. Even if I possess only one of a deck, I open it and use it - I treat it gently, but I do use it. What the point of all that premium quality and design if it never leaves the box or ends up in my hands and on the game table?
As much as I like performing with cool-looking decks that no one's likely to have seen before, I usually keep it simple and get something cheap when I'm performing at a bar (a dirty bar + rare cards = paper recycling) or at the hospital (messy kids eating greasy food their parents brought in to replace the hospital diet = a dirty bar, see previous formula).
I'm going to bust out my new LTD and Green Artifice decks and play some solitaire after I'm off the board!