They've certainly very subjective and interpretative.
I don't think there is much of a difference from looking at their shapes, though arguably there is from looking at their lengths (which relates, in a sense, to their "shapes", perhaps?).
To me, a "pennant" is a short symmetrical triangle. Up to a certain (variable) length, people call them "pennants", but above that length, they’re often referred to as "symmetrical triangles" or "rising/falling wedges", aren’t they? I'm not particularly impressed by the usefulness of these chart patterns, I have to say - and especially pennants that slope in the same direction as the general price trend seem to me to be of very little, if any value.
There are probably some good examples on Thomas Bulkowski's site? (thepatternsite.com, if you haven't seen it.)