Which visa do I get?

For the 1st time transit, as you'll go outside the airport, you need a transit visa.
For the 2nd time transit, if you'll stay at the airport transit area and the whole transit will be done within 24 hours, no visa is needed and a single-entry transit visa is OK for your twice transit in Beijing. Otherwise, you'll need another transit visa. In this case, you need to apply for a double-entry transit visa all together.