They are technically equal. However, a high school diploma, implies that you did complete four years of high school. Better than a GED or high school diploma, is having some kind of ongoing education following either, such as, but not limited to a college education, some tech school training, etc. : ) It always gets you a little further on your resume, when future employers are reviewing you as an applicant.
I would have to say diploma because my dad had to get his GED, and when you apply for a job at your interview or something they ask you why you got your GED and didn't stay in high school but hey that's just my opinion hope that helps.