Good advice, thank you. But what does CDX mean?

CDX is the grade of plywood. One side grade C, the other side grade
D, the X is for "cross this off the pretty list." Basically, the
crappiest stuff that still qualifies as plywood.

The other concern about plywood,
while I did consider it, is those holes to be drilled for hold-downs and bench dogs. I can
imagine that pressure will be applied toward the sides of the holes at the lower ends,
eventually that ply will begin to break down and chip?

It'll probably be awhile, but when it does happen, drill out the
holes, tap some dowels into the (now larger) holes, and re-drill your
dog holes. I did the same thing in a SPF (Spruce/Pine/Fir) bench with
an MDF top.

I can also imagine DF 2x4's will
also develope cracks because of the wide grain structure. I heard that poplar is only a
little harder than DF (much local) but has a much better grain, tighter, and lower cost.

Alex

I can't imagine anywhere in America that poplar would be cheaper than
construction-grade lumber, but what do I know? Don't answer that. I
would give serious consideration to Jummywood (Southern Yellow Pine).
It's only slightly more expensive than white wood and twice as hard
and plenny dense for a workbench.

As always, YMMV.

-Phil Crow