Again, I would strongly suggest borrowing/trying if at all possible. The DSS and Halcyon plates are very different - so much so that I replaced my DSS (medium) plate with a Halcyon small plate even after I had replaced the harness on the DSS. To be fair, however, the reasons I like the H plate are reasons others prefer the DSS... Which is why I suggest trying them out if possible. (Same thing is true with drysuits).
If the cost is a concern, but you end up with a preference for Halcyon, I'd make two points: 1) it's a heck of a lot cheaper to buy the right thing (for you) once. 2) second hand H short plates, especially SS, do come up for sale periodically, and since they are not a wear-and-tear item, they are a no-brainer to purchase used.
Just a few more thoughts.... From someone who DID buy twice