Well IMHO there should be no such thing as a "default" set of datum transformation parameters for a datum, and ArcGIS and OGR are both wrong in allowing the user to use a default transformation without further prompting. It just so happens in this case that the one OGR uses (NADCON) is the most accurate available for the area covered by your data, but in the general case (worldwide) this is very hard to determine and there normally is no such thing as a default. The transformation that should be used depends on the exact area covered, accuracy required etc.

In most cases in GRASS, when choosing projection parameters, the user is interactively prompted and forced to make a decision on which datum transformation to use. I feel this is "a good thing" because, in the best traditions of the steep learning curve in GRASS (!) it makes the user think about the meaning of their data and how reliable they expect the result of the processing to be (garbage in garbage out etc.).