what is the constitutional debate between state vs federal power when it comes to lower the drinking age?

The federal government does not set the drinking age. The states do. But, in 1984, the federal government passed the National Minimum Drinking Age Act, which would penalize any state that did not have a drinking age at a minimum of 21 by withholding 10% of federal money for transportation.

This was challenged up to the Supreme Court (South Dakota v. Dole), who ruled that the law was constitutional under the Taxing and Spending Clause of the US Constitution.

So there is no debate. It's been settled a long time ago. States are allowed to set the drinking age to whatever they want, but if they lower it below the age of 21, they will receive less money from the federal government for transportation.