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.