Working in an auction venue, the "plus" can add a bit of value to an otherwise-normal coin, but it almost never seems to push it "half way" into the next grade range's value. The STAR, however... that's a whole different ballgame. I have seen some coins with a very mundane grade get pushed FAR above the grade-range's typical value. The best example I can think of might be a common Morgan dollar, say MS-63, that sheets for $75 but sells for $3,000! Of course, one might say that it was because of the astounding bag toning, but the point is - THAT coin earned a "star" and not a "plus" because of the unfathomable eye-appeal, not because it was "close to MS-64".
In that regard, I believe the "star" is the more desirable designation.