I would say PEK. I've never been there... but I know TPE well....

One reason I can think of - PEK is changing very rapidly, TPE had its boom years. Sure new buildings are still going up - new underground lines are being dug. But I reckon you could do TPE in 10 years time and it won't be much different. I doubt that that could be said about PEK.

Besides, I just think there will be more to see than in TPE.