Both airports about about an hour--TPE a little less, by bus (lots of buses to various locations in the city) or taxi, though exact time depends on traffic. PEK has a train option to a reasonably downtown site (where you can get the subway), but taxi is pretty cheap too. In a word, Beijing is just much, much, much bigger than Taipei, with more things to do and certainly more historical sites. The dollar goes farther in Beijing if you're not spending it in Western-standard hotels and restaurants; otherwise, I reckon they're about the same. Might as well spend your dollars before they're completely worthless.