What I know about plating is from the hydraulic cylinders that I have used in the past.
1) I can't think of a reason that the substrate would make a difference. On the rare occasions that I have used stainless steel with a chrome plating, it has been to prevent corrosion of the un-plated portions.
2) It appears that your project requires an aesthetic quality. You stated that you need a "shiney looking wear surface." Speak directly with your vendor and ask to have a look at some final parts to make sure that it's shiny enough for what you're looking for.
3) In the industry, a "hard chrome" surface is not the same as a chrome-plated surface. A chrome-plated surface is not as hard or wear-resistant as a hard-chrome surface. The hard-chrome surface is probably what you're looking for, but what may be an issue for you is that although it is pretty shiny when polished, it is not going to be as shiny as the bumpers on your dad's '58 Buick.
4) The problem that you're probably going to run into on this is that dirt, grit and contaminants will get between the sliding surfaces and start to scratch off the coating, even though you're talking about an incredibly small surface pressure (0.08 psi). With that in mind, I don't think this is really going to be an issue of trying to decide what type of plating you're going to use. You might consider using a lacquer coating on top of it and go with the assumption that the lacquer will wear off and require replacement from time to time. Will your setup be lubricated? Can you put a wear pad under the aluminum block? If you use Nylatron (Nylon impregnated with molybdenum disulfide as a lubricant, cheap and readily available everywhere), I doubt you would ever have to worry about any wear at all.
5) All that being said, what you're talking about is a matter of simple hardness, i.e., which finished surface is harder? There may be a nickel surface treatment that can come close to the hardness of a hard-chrome plating, but I don't know of it. If you're looking for just the hardest surface you can get, go with a 0.005 - 0.010 in. thick hard chrome plating. You're probably not going to get much harder unless you go for a tungsten carbide plasma spray, ground, honed and lapped.
