Taiwan invasion definitely. They have a strong incentive for that because uniting China has always been CCP's (and historical dynasties) main priority since the founding of PRC. While nine-dash line claim is merely a geopolitical interest for expanding their power.
What we can be sure of is that it's impossible for PRC to press SCS claim without conquering Taiwan, because Taiwan will surely be used as a US base of operation if they do invade the south. So leaving Taiwan unannexed, will leave China open for the US maneuvering in their front door. And because Taiwan also claims SCS, PRC legitimacy will be challenged if they didn't incorporate Taiwan first, providing the US and allies all the more reason to defend Taiwan.
The conclusion is that China would not invade the south if they haven't conquered Taiwan, so conquer Taiwan first, only after that SCS and ASEAN countries. While at the same time if we assume they are busy with Taiwan, occupying themselves with Taiwan affairs will draw their attention away from SCS, buying time for ASEAN to prepare against or negotiate with China.
