Chiming in here with my admittedly somewhat biased opinion as Tempest is my favorite game of all, period, end of story, so keep that in mind as you read on.
Between the two games and if you must choose, Tempest is the better game overall and will hold your attention for a longer period of time, IMHO. It more than fulfills the classic game mantra of "Easy to learn, difficult to master" and will likely keep you very entertained for quite a while as you learn how to get deeper and deeper into the game. You'll need to learn strategy and quickness to master the game, particularly if you do get all the way to the black ("invisible") and then the green levels. And there are no patterns involved...but again, it's all strategy and quickness. And I really like that, no matter how good one gets at the game, the sheer randomness of the various enemy's movements and actions will eventually get you.
For perspective, even after all of these years of playing this game (literally from when I saw my first upright when it was originally released BITD), I can play pretty much forever if the extra man settings are for 10K, but on factory settings (extra man every 20K), the game does get me eventually

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As for SW: It's also certainly a very good game and more than worthy of classic game status, but overall it is a little repetitive after you play it for a while. But on the other hand, its collector/monetary value is higher and will likely remain high what with the seemingly never-ending movies, TV shows, and merchandising that will no doubt keep many generations to come interested in this game.
I recently brought my upright to my son's school for a charity event, and the kids could not get enough of it...it was in constant use and even the parents got in on the action a bit (when they could elbow out the kids ever so briefly

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Of course if you do get SW, be sure to get the ESB kit...not as good a game as SW, but still fun to play sometimes. Someone needs to modify that kit so you can have an option to earn back a shield

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Anyway, there's my 2 cents.
If I were in your shoes, I'd do what I can to pick up both if at all possible...you know you'll want both eventually anyway

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Jon