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?
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I believe printing directly on aluminium is unwise, simply because it will expand when heated, typically giving the bed a concave or convex shape. Glass, on the other hand, does not (at least not significantly).
As pointed out in the comments below, the heat expansion of aluminium could potentially be mitigated by increasing the thickness of the bed, as well as heating it evenly. Also, a common solution is to place a glass plate on top of an aluminium bed, at the cost of a slightly longer heat-up time.
In my experience, printing directly on heated glass can be very practical and give a nice surface finish for some PLA variants and other materials that support it. I don't know if printing directly on aluminium can give similar benefits.
I use Kapton on top of it, and the print sticks easily with PLA. ABS requires hairspray on top of it, and nylon requires glue stick (but it fails anyway if the piece is big).
If you use glass with a top layer for proper adherence, you should be fine. Aluminium below could help spreading the heat, but... it will bend and push the glass, therefore you increase the risk of breaking: the glass does not bend much, but internally it is ALREADY withstanding a lot of stress when hot, and aluminium below could push it to the limit.
Edit: it appears however that aluminium beds are in general the best ones:
Aluminium gives low adherence. Probably because its top of surfaces gives temperature fast. But using conductive metals is the right direction. There are some benefits:
I tried titan. Here is prooflink. Using 0.5 mm it gave good results on printing with temperature 120 degrees Celsius. But as I had bit crooked plate, I had to stop using it. Needing to heat it up 20 degrees more than glass makes it pointless, unless using microwave. Try borosilicate glass.
I have only printed on an aluminum build plate but I agree with Craig Anderson. I use Marlin RC6 (I really should update to RC7 but I am lazy and RC6 is working well for me) and the mesh bed leveling system. WELL worth the time to get it setup. I use blue painter's tape and I print in both PLA and ABS, but mostly ABS. I have very few issues with the part(s) not sticking to the build plate, actually the problem I have tends to be the opposite LOL! I started to get good, repeatable results when I switched to mesh bed leveling and then great results when I changed my first layer to be 200 % flow. YMMV but I print with 110 °C bed and 235 °C print head, use blue painters tape and mesh bed leveling.
I've got a full frame i3 with a glass printing surface, and an Anet 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.
I have used glass from the beginning and it works perfect. (From old picture frame and old scanner)
I have tried glass with tape, glass with hairspray and have had very good luck on printing on both. Prefer hairspray :)
I have tried printing on the heated aluminum, but experienced the aluminum plate tend to warp due to temperature changes.
With a glass plate on top of the aluminum, you have a flat surface that don't warp. :)
So I would give glass thumps up!
EDIT: Printed on Glass with glue-stick. Wow. Sticks really good.
I use a 200 mm x 200 mm aluminum bed over an aluminum heated plate. It is attached by bolts in the corners. I use a proximity probe to run a leveling program and I print on the aluminum. I don't see where my bed is bowing or causing attachment problems. I run the leveling program with the plate at temperature (110 °C) and any bowing is handled by the software. It works very well with ABS. I haven't used much PLA and am still experimenting.