Global variables require a harddrive write/read
local variables are held in ram. Also, by avoiding
set variables as much as possible you 'future proof'
your scripts come the day you wish to run it off a
solid state harddrive or flash memory device. I try
to only use a set global variable if I need to retain
the data across sessions and make it available to all
scripts and connections. Another nice thing about using
local variables is less likely to have variable name
collisions. They only work within that one alias, event,
or menu item. Makes it easy to use single letter variable
names.