Table index generation


Visio offers four index types for database tables. The two of interest to me are:
Unique index only
Unique contstraint only
I have created one of each in a test table in my diagram. When I forward engineer, both are scripted as a "CREATE UNIQUE INDEX". I expected the "Unique constraint only" to generate a constraint, not an index.
While both can get the job done for data integrity (with subtle differences), it is problematic if I start with a reverse-engineered db, which uses constraints. I get a lot of "false" differences just by forward engineering the db, without making any changes to it, because the constraints become indexes.
I realize that I am choosing what Visio calls an "Index type", and I am expecting a resulting contstraint; but, what is the point of offering me two choices that give the same result? Is my issue with Visio or the Forward Engineer add-in? Any help would be appreciated.
, Lee