1/2 inch or 3/4 inch plywood?

[ARTHILL]

I have used both, the choice being what I had on hand, or what I could scavange. When I have to buy it, I use 1/2 because it is easier to cut and work with. Both are strong enough for HO and spans up to 2 feet.

[Bill54]

I'm getting ready to start building my HO layout and need advice on the thickness of the plywood that would be practical for my layout.

The layout will be a cookie cutter type with L girder benchwork.  All along I have planned to use 1/2 inch plywood for the sub roadbed.   But now that the time is here I'm questioning whether or not I should use 3/4 inch?

The layout will be in an E shape 15' X 20'.  It will have a lower level for staging.  To get to the lower level the trains will run around a double track that will be attached to 6" wide plywood.  It is this plywood that I'm in question of. 

I know the experts here have already gone through this dilemma and have reasons for using one over the other.  Please chime in and let me know your reasons for either.

Thanks in advance for your guidance.

Bill