Using rems would:

• Allow us to use relative sizes and prevent the CSS compounding effect
• Have us need to define the root selector's font size (although, there's the possibility to define our own global CSS)
• Improve accessibility by adjusting to the font size defined by the user

Using pixels would:

• Prevent the CSS compounding effect
• Prevent us from having to define the root selector's font size every time
• Override the user custom font size set up