Depends.
Portal: Subtle humour, 1st person puzzler. Emphasis on thinking outside the box and character interaction.
Duke: In your face humour. 1st person kill-a-thon. Emphasis on Duke loving himself and the ladies whilst shooting alient pig people.
Both very different games but both very fun. Its what type of game you fancy playing as they are different experiences.
To be honest with you I wouldn't say Portal 2 is worth £30, and I have no idea about Duke.
I'd wait for the sales.
Portal 2. DNF is terrible.
DNF is a 10-20% game its terrible even at budget price!
Yep a lotSo its went down from you initial estimate of 40%?
Why not, I thought it was a really long game compared to first one, I thought it had everything the first game had and a whole lot more, and I thought it was one of the best games thats ever been invented...everyones seems to love it...so many rubbish games come out all the time at £40 and upwards, portal 2 is only £18.88 now on amazon for PC.
Graphics/gameplay = 10/10 - Also probably most peoples GOTY.
Took me a week to complete it, played like anything from 4-6 hours each day.