you only list a single first and single second stage, not what the whole set is
so pros and cons.
I don't like piston first stages because they require extra maintenance to seal if they can be sealed, and you have to be more diligent with them during cleaning after diving because important stuff is exposed. Nod to the FDX10
I much prefer swivel turrets, especially those with a port on the bottom for all primary first stages. Nod to the MK20 *they did have a recall for some stuff a while back though, so make sure when you take it in, that all recall work is done. I think it was just the washer between the yoke/din turret and the main body, but just have it checked out by a dedicated scubapro shop*
I think the S550 is going to be a better second stage, and while it can be upgraded to the S600, I don't think it's necessary *replacing plastic bits with metal bits*. You can use the Delta as a secondary with no problem and the second stages can bounce around to either first stage.
Personally, with this info available, I'd use the FDX10 first stage, with the S550 as primary, and Delta as secondary. Depending on if you have a scubapro secondary, and what it is, I may suggest that as your secondary, but for single tank diving, I think the FDX10 gets the nod from me because it is sealed which means you can pay a bit less attention to it, and if you're travelling it is smaller/lighter. Keep the MK20 because it has better hose routing imho and you may decide you prefer that. If the second stages and first stages are tuned properly, as a diver you shouldn't be able to tell the difference.