I want a revision of my chin augmentation. What type of implant should I choose? (Photos)

The chin implant that was chosen for you may the correct size and shape. Migration of the implant after placement is possible. Most surgeons secure them with sutures or screws. I prefer screws. 

If you feel that a revision of your chin implant is needed, it is possible that the same implant could be used but just placed lower along your jaw.  If you are looking for more augmentation on the sides, there are a variety of implants that may be longer than your current one and provide this side augmentation.

With such a high chin implant position you may not necessary need a new chin implant...but the existing one positioned lower. It would be impossible to really know how the shape of the current chin implant is when it is not in the best position on the bone.

Matthew
The Xray shows the implant to be a bit high.  While there are a variety of materials used for chin implants, silicone is, by far, the most common.  If  you were to undergo a revision, my advice would be to simply reposition the implant that you have.  To do this, the surgeon would need to fix the implant in its new position.  This is commonly done using very small titanium screws.

I would quite agree, The placement is definitely not ideal. One could reposition it and use a screw to fixate it properly, change it (less necessary) or do a sliding genioplasty which keeps your own chin shape and therefore is more natural in its contour.