I've still yet to beat Quantum Conundrum so I can't really say how the quality or length compare to Portal, but definitely play Portal. Both of them are fantastic and kinda in the same sort of 'do puzzles while being talked at by some condescending voice'.
I can't remember how long Portal 1 took on my first go... but now I can kinda wiz through it pretty fast (not like... Gamefaqs user god of every game fast, but decently fast at an hour or something (maybe 2... that may be stretching) with trying to break the game here and there). It's still a fun ride each time and something I love picking up when I have a bit of time.
Portal 2 is longer (I'd assume it's longer than QC also... but I dunno how long that game is... also I don't tend to time my playthroughs...). I want to say 6 to 8 hours for a first playthrough... maybe... I dunno. But it's also definitely worth the time. It improves on Portal 1 in just about every way (though it is a fair bit easier). The new mechanics work nicely and the story is just fantastic. Portal 2 is definitely one of my all time favorite games.
Definitely play 'em.
This user is awesome!: http://boards.gamefaqs.com/gfaqs/user.php?board=927246&topic=44002798