https://forum.cmascenter.org/t/biogenic-emission-model-megan-or-beis/431?u=skunwar
The EPA uses BEIS for its analyses, and BEIS is integrated into SMOKE, so BEIS is sufficient.
MEGAN is also used by some parties but draws on different land use databases.
You may want to research past applications of MEGAN and BEIS in your region of interest to decide.
Alison is right that BEIS should be sufficient if you are modeling the North America regions. If you are modeling outside of NA, then MEGAN will be a better choice for you.