I love deba threads...
So I started with a regular deba a couple years ago and after I picked up my mioroshi, I never looked back. From a practical standpoint, the regular deba really only has a couple of advantages -- heft (regular 7-9mm wide above the heel, 4-5mm for mioroshi) and if you like tall knives, height (50-60mm vs. 40-50mm). That extra beefy-ness makes going through spines, taking apart heads, and gliding through ribs a breeze. This is noticeable even in small fish.
What the mioroshi lacks in brute strength, it makes up for by being much more nimble and able to handle many more tasks related to seafood that would otherwise be way too clumsy for a regular deba, like removing the skin off a filet. Because of this, most fish cooks that use a normal deba will switch to a gyuto or even a yanagi when it comes time to clean up their filets and portion them.
And while you can absolutely force the issue, I find the mioroshi is better suited to dealing with its shortcomings than a normal deba. With a microbevel, my mioroshi's been able to handle spines close to an inch thick.
So based on what you're going to be using it for, the mioroshi gets my vote. As for the steel, it really depends on your own preferences. I like them in carbon steel, blue or white #2, mainly because stainless and single bevel isn't something I can wrap my mind around. But, if you go for stainless, Suisin makes one that I'm sure is awesome. And expensive. But also awesome.