Charge the MacBook Neo battery - Apple Support
Charge your Mac laptop computer - Apple Support
Use a power adapter with your Mac - Apple Support
Either adapter should be appropriate. Apple mentions the 20W adapter by name, but the 30W one should work, too.
The Mac and the power supply will negotiate the charging voltage to use, and the charging circuits in the Mac will regulate the amount of current that the Mac draws. So if the Mac willing to accept 30W of power, it might charge faster with the 30W adapter. But if it's not, it would just draw what it wanted (e.g. 20W of the 30W capacity).