Over by Westport.
The platform or step one is just that. Pair of hooked aluminum tubes that hook over the gunnel. The down tubes have a number of holes drilled in them.
A set of holes in the tubes welded to the carpet topped aluminum platform so it can be adjusted up or down.
The 'swim in' type 'usually' have some sort of inflatable bladder with a fabric or plastic ramp that extends down into the water. It does not take long for the dog to figure out where the opening is and they start swimming into it.
They swim into the opening and walk up the ramp and into the boat, or as with ours, there is a platform at the top they can sit on or use to get that to get in the boat.
Saves them a LOT of energy over climbing on a platform. Particularly if there is ice and they are cold.
Other than size, I would not go back to a platform after using the swim in type, but the platforms do have their uses.
I can send pic's of the platform if you are interested.