acid alpha glucosidase vs glycogen phosphorylase
Acid alpha glucosidase  (sometimes called α-1,4-glucosidase or acid maltase) is not among those enzymes involved the pathways of glycogen synthesis and breakdown. It however occurs in lysosomes,where it functions to hydrolyze the disaccharide maltose  and linear oligosaccharides, as well as the outer branches of glycogen,thereby yielding free glucose.

This "second pathway" of glycogen metabolism is not quantitatively important as it concerns a minor fraction of glycogen. The reason that lysosomes normally take up and degrade glycogen granules is unknown perhaps for the disposal of structurally aberrant glycogen molecules.



Glycogen phosphorylase (a dimeric enzyme)
catalyzes glycogen phosphorolysis (bond cleavage by the
substitution of a phosphate group) to yield glucose-1-
phosphate (G1P).

$\ce{Glycogen + Pi &lt;=&gt;  glycogen + G1P}$

This enzyme will only release a glucose unit that is at least
five units from a branch point. (Note: I am not going to describe the reaction mechanism of glycogen phosphorylase here is much more complex than it sounds)

Having briefly described the enzymes, the most important thing to note is that:


structually these two enzymes are different, 
active in the different locations 
the mechanism of glycogen breakdown are different (i.e phosphorylation, hydrolysis)
different regulation mechanisms