Ultimatums come in four alloy flavors; AEB-L stainless, Bohler 390 stainless metallurgical powder, 52100 carbon, and HD semi-stainless.
The HD knife is somewhat different than the others. It's made OEM for Richmond by Konosuke in Japan, while the others are made OEM by Lamson-Goodnow in Buckland, Massachusetts. According to Shaun Fernandez's video review (posted at the CKtG site), the HD Ultimatums, which are closer to 300mm than 270mm, are more like Konosuke laser gyuto than they are like the other Ultimatums. The other knives are not only shorter -- which are not only shorter but significantly stouter.
Shaun knows what he's talking about, so I'm perfectly willing to take him at his word. Also, on the basis of a great deal of experience, I can confidently say that a 52100 Ultimatum and a 270mm Kono HD laser gyuto are VERY different knives. In my kitchen, they compliment one another very well to the point that my carbon Sabs -- excellent knives in their own right -- cannot compete at any task.
52100 is an excellent all around carbon alloy, in the same class of excellence as any of the excellent Hitachi YSS carbon alloys -- Shirogami 1, Shirogami 2, Aogami 1, Aogami 2, and Aogami Super (aka "AS"). There are other carbon alloys in the same class of excellence as well.
52100 was originally made as ball bearing steel. In the past few years, some changes have been made to its formulation to make it extremely well suited for knife making as well. It is made under a variety of proprietary names by a few different foundries in Europe and the US. I believe, the 52100 used for the Ultimatums is made by Latrobe in the US, but am not sure.
AEB-L is an excellent "strip steel" stainless alloy made by Uddeholm. It's also made by Sandvik as 13C26. It's primarily known for having edge-taking properties equal or better than any other stainless. In the Ultimatum, it's hardened to 61, and is at least "very good" in every knife alloy respect -- including edge holding.
If you sense a however coming, you're right. It's the Bohler 390 Ultimatum. Bohler 390 is a powdered metallurgical stainless. The 390 Ultimatum will take as good an edge as the AEB-L version, but hold it much longer. It is also more resistant to impact burring. In my opinion -- based mostly on what I've head from others -- the 390 version is worth the extra $50 compared to the AEB-L.
I chose the 52100 version for myself because of my interest in carbon alloys in general and because I was extremely curious about that particular alloy. Like many carbons, 52100 has a very smooth and pleasant feel on the stones. It has better edge taking characteristics than the dated carbon alloys of my Sabs, and much better edge holding, corrosion resistance, resistance to impact burring, etc.
Because of it's outs outstanding edge holding qualities and because it's stainless, 390 is a better practical choice than 52100. But I can compensate for whatever 52100 gives up to 390 by using a steel now and then between sharpenings, and with slightly more frequent sharpeneings; and since 52100 feels better on the stones it was the right choice for me.
If not exactly "mighty gyuto," the non-HD Ultimatums are as robust as forged European knives, and up to just about any ordinary kitchen task short of splitting mammal bone. However, they are heavier and balance further forward than most other wa-gyuto. I'm in the process of writing a review, which will go into the knife in detail, and which should be done in a couple of days at most.
Undoubtedly AS would have made as good an Ultimatum as 52100, but an AS knife would have cost significantly more -- not only because AS costs more in bulk, but also because it's more difficult to work and because it makes for a higher percentage of failures -- especially as a single steel.
AS doesn't have a particular look. I'm not sure which knife style you like, but whatever it is, it's probably available in a lot of different of alloys. AS's claim to fame, is that it's tougher than the other two aogami steels. The aogami steels are supposedly less chip-prone and more corrosion resistant than the shirogami steels.
Unless you're an amazingly good sharpener, you won't be able to get a Shirogami #1 knife any sharper than a knife of similar geometry and similarly appropriate hardening made with another of the many other excellent alloys. As you can deduce, the same can be said of any of the excellent alloys.
There are so many alloys because each has its own strengths and weaknesses. Those can be enhanced, limited and/or compensated for by a number of techniques, especially heat treatment and geometry; but their are some alloys which are either too limited or too expensive for knives at given prices. For instance, SK4 would be inappropriate for a $200 knife, and White #1 would indicate a great many compromises in a $60 knife.
Knife maker have their own reasons for preferring one alloy over the others. Those may or may not mean a better knife for a consumer.
At the end of the day, the gestalt of the knife -- including its appearance -- is more important than any particular alloy.