all games are not created equal

I adore both games, for sure.  While I feel Mirror's Edge is underappreciated for what it set out to do, it's execution of the idea was hindered by some bad level design choices later on.  Portal never really set out to do much more than it's basic concept of creating doorways anywhere you want, but it stuck true to itself by never trying to suddenly change that concept, such as suddenly giving you a pistol for some reason.

Mirror's Edge gameplay mechanics was well thought out and executed well.  Seeing pipes, wells, fences, and zip lines as a means of travel really makes you feel like a ninja.  However the game suffers once it traps you indoors in warehouses and subways.  Roofs are excellent, and even shopping malls can be fun.  Offices and tunnels aren't.
The story mode was ok, though I didn't really care about the story.  My favorite part of the game is the time trials, where you simply race against the clock to collect checkpoints and try to find the shortest/quickest route to take.  This is really more like what the story mode should've been.  The DLC level pack also looks amazing, and should be a treat to play.

Portal is hilarious all the way through, and it's kept nice and short without overstaying it's welcome.  Packaging Portal with the Orange Box is a very safe and effective way of trying a new concept.  If you don't like Portal, then you still get Team Fortress 2, HL2: Ep 2, and plus Ep 1 and regular HL2.  And if all you're looking for is Portal, you can probably nab it on it's own for less than 20 bucks.

Both games are great, and Mirror's Edge may eventually evolve into a fantastic game, but Portal's charm and humor has no equal to me.

Having just finished both of these games for the first time, I am fresh with ideas. While both games do some revolutionary things for a first-person perspective driven game, Portal is the superior game due to better gameplay, a truly unique and interesting style, and a deep, mysterious story unveiled through nuanced delivery.

Mirror's Edge does score some points for large, beautiful environments, and is certainly the graphically superior game, but the deeper style of the game's overall environment (tyrannical government) can feel a bit tired. Also, the control response in Mirror's Edge can be a bit flaky.

Mirror's Edge shines when considering replayability and multiplayer mode. Each level has multiple paths to explore, things to collect, and time trials, not including available downloadable content, almost all of which you can play with someone else. Portal has a few extra modes available after you finish, but, like most puzzles, figuring out the solution tends to be the end of the line.

Both games offer unique experiences, and should both be played through, but if you have to only get one, Portal's challenge and charm will infinitely satisfy you.

And it's cheaper.