6061 is much more common, the price / availability will be better.
A few observations of my own, you might know all this but still:
* To improve both strength and heat spread, use a thicker gauge plate.
* Concentrate on getting a good thermal connection between the glass plate and the aluminium too - any air in there will counteract all your hard work heating the aluminium evenly. Best option in my mind would be to have a super-flat, smooth surface on a thick alu plate (to minimise warp), and some well-spread clamping pressure + smear of thermal grease between the glass and it. - think like you're doing a computer heatsink!
* Also, thinner glass heats up faster, and shows less temp diff between the bottom (measured) surface and top (working) surface (due to its relatively high thermal resistance and the net heat flow through the plate), but is of course more fragile. Stick to about 3mm (most plates for printers are about this size). Borosilicate is recommended as due to it's super-low thermal expansion coefficient it's far less likely to crack/distort when heated.
* Insulate the edges of the stack, I think a fair bit of heat comes out here which contributes to the cold edges.
SD3. Mk2b + glass, heated enclosure, GT2 belts, direct drive y shaft, linear bearings, bowden-feed E3D v5 w/ 0.9° stepper
Smoothieboard via Octoprint on RPi