Plate material, i.e. Stainless vs lightweight (Kydex or Aluminum) is a for me a function of required ballast. Warm salt water implies thin wetsuits (~3mm) and steel 120's are typically about -2 lbs empty.
If we start with a neutral diver add 3-5 lbs of buoyant exposure suit, offset with 2 lbs of empty cylinder and ~2 lbs of regulator you can see that the ballast provided by a SS plate will over weight the diver. Even with a lightweight plate there's a chance the diver may end up very slightly over weighted.
With alum 80's the stainless plate starts to look better, as alum 80's are ~+4 lbs empty vs -2 for most steels. That 6 lbs difference make the ballast an SS plate provides welcome.
Keep in mind that the dry travel weight difference between the typical SS plate and Lightweight plate is about 3 lbs. many people assume it's more like 10 lbs. It isn't.
It's also the case that even with a SS plate often a BP&W, (with a smaller Warm water wing) is lighter (travel weight) than many full featured BC's, or at most 1-2 lbs heavier.
Tobin