What's the difference between BTU and CFM when comparing air conditioners?
They have almost nothing to do with each other. 

BTU describes the actual cooling power. 

CFM simply describes how fast it moves the working fluid (the air) through the system, and CFM does not matter for differences that small. Suppose you order water and one vendor supplies four 3-gallon jugs and the other supplies three 4-gallon jugs.  Not a difference enough to care about. 

Noise has nothing to do with either one, and everything to do with the acoustics of the machine, shape and design of the blowers, and isolation of machinery.  Slightly more air volume matters little by comparison.