How to choose laser galvanometers
The galvanometers are rated in terms of how many different angles they can point at per second. The smaller the angle the mirror has to swing through, the more times per second it can do it, which is reflected in the specs for the second galvanometer you pointed to. For a light show application, the desired patterns and level of detail would determine both the required deflection and the number of points.

The generic component selection process is applicable here and goes something like this:


figure out a performance requirement for the component
find some components that meet the requirement
cheap enough? you're done!
too expensive? try adjusting your design to work with a cheaper version, go to 1.