Get them both. Worth it.
If the sequel is anything like the first, then definitely buy it. I just got Witcher 2 and Portal 2 however, so I'm going to wait and see what else they put up.
Portal 2 for under $10 has been the steal of the year thus far. I paid $50 for it day one, and it was worth every penny.
Well I guess to each his own, but as much as I loved the original Portal, it was 20 bucks and that was the most I'd ever pay for it. Unless Portal 2 is two and half times as good I'm glad I never bought it fully priced.