The difference is conceptual... it wont affect the way you run calculations nor the definition of spread.
If you define unit as your minimum bet, then the generalization of the definition of unit would become very vague and very difficult to use.
Player A spreads from $5 to $1000.
Player B spreads from $5 to $50.
According to the popular definition of unit, they both are playing with $5 units, but wouldnt you agree that talking about units as a general concept in this case would be useless from any analytic perspective?
If you look at both players ramps you could reach to a much precise definition of unit that would allow you to consider the difference that exists between the 2 players.