Glass is always level, easy to clean, easy to work with.
Aluminium allows for the addition of automatic bed leveling with an inductive sensor and distributes heat a little more evenly.
When printing mostly ABS and PLA, which one is better?
I've got a full frame I3 with a glass printing surface, and an A8 with an aluminium surface.
According to my optical thermometer there is a 15 to 20 C temperature loss between the bed sensor and the glass surface at 65 C, which is how hot I have to run the I3 to get adhesion with PLA.
I have not tried printing ABS yet on either printer, but I cannot see getting the glass surface temperature high enough to give good adhesion.
I use dilute PVA adhesive as on both surfaces, but it does not adhere well to the aluminium.