They both produce .vsd files that are compatible with each other.
HOWEVER,
Pro contains extra functionality that manages meta-data behind the drawing
that standard doesn't have. For example, I think Pro contains UML and
Database solutions that Standard doesn't. When you open a UML diagram in
Standard, it will be somewhat "dumb", because the UML code won't be their to
give you advanced features.
You'll be able to move shapes around, print, delete, etc. but the higher
value of the drawing will no longer be there, and the UML metadata behind
the drawing could be "corrupt" so to speak.
--
Hope this helps,
Chris Roth
Visio MVP