Dipladenia and Mandevilla – How They Differ

Both Dipladenia and Mandevilla are beloved for their brilliant, showy trumpet shaped flowers in shades of red, coral, pink, yellow and white.  These evergreen, tender, tropical plants flower profusely in summer to late fall/early winter in the Bay Area and attract hummingbirds and bees.  They look alike – so what are the differences between them?