I was using a Droid X for a couple of years and love that phone. I upgraded to the HTC One X and after the S3 came out, the wife has the One X and I have been using the S3 for a year now.
Do not hesitate IMHO at least. The S4 will be an awesome upgrade from the Droid X
S3 to the S4 ???? I am still trying to make that decision. Will go to the AT&T store and check it out when it is released. The fit, feel and screen will be my final decision