Both the 175 and 90 can be attached through the hull and the 175 is the only one that can be mounted to the hull. Personally, I am more comfortable with through the hull mounting to a backing plate. The 90 backing plate is threaded which makes it very easy to install. The 175 backing plate looks the same as the MightyMount backing plate http://www.yakattack.us/Images/ProductPhotos/FullBack/FB-GT175.JPG
Outside of the width, the main difference is the number and location of the attachment points. The 90 has the screw holes in the center and the 175 has bilateral screw holes along the length of it. Will all other things being equal, the bilateral screws on the 175 are likely a more secure connection given the number and location of the screws. With that said, I have used the 90s exclusively and I feel that they are more than sufficient for the loads that I apply to them. This includes attaching Hobie Sidekicks to them for sailing and standing (and sometimes lifting the kayak by the Sidekick's center bar).