The ST500 and ST550 are known to be point and shoot cameras that were released a few years back. One main difference between the two is that the ST550 is known to be a bit more expensive as compared to the ST500. One main difference between the two is that the ST550 will have a bigger rear as compared to the ST500.
Another main difference between the two is their weight. If you want something that is more lightweight, the ST550 can be the better option available. Back then, point and shoot cameras were still new, and people found it really awesome that they can immediately see how they look like. Of course, this is something that is completely new right now.