hynbae's black Ironman is a 1989, and was rumoured to have the twitchier geometry.
There is absolutely no printed data I could find to support that, other than an article that said Centurion said...etc.
The opinions of two others who own both a black model and another model, unsolicited, are all there exists of any mention...

...Besides my opinion, having owned both a 1989 Master and a 1989 Expert at the same time.
The Master, with 2x10 105 and Alex ACE 19 wheels (with the 10-sp only Formula/Zipp/DA hubs) was the smoothest bike I've ever done a century on. The Expert, with 2x9 105 and Sora shifters, Veulta wheelset, was the lightest and twitchiest Ironman I've ever been on, and certainly felt more skittish than the Master. It didn't come with a fork, so I used CF. I'd not have taken it on a century; too skittish.

CardiacKid's is also a 1989, but I'm not sure if it's the blue smoked model or the all-black model. His comparison to his RB-2 has validity, since he owns each one. I've ridden an RB-1, but not enough to make a comparison. I can't say I preferred it to an '89 Master, but it was a very very limited ride.

Properly set up, an undamaged Ironman frame is an excellent all-around machine. Comparisons should be made with identical wheelsets and with approximately equal components, and the same fit. Just as with any other bike. There were several Centurion models that could be considered a bit more agressive than the Ironman. The Prestige, which I've only seen in 1985 mode and the blue 1989 beauty on this thread, was a lighter, almost "petite" feeling frame that you could really, really hammer on. The 1986 Facet was aluminum, hard-edged, and I felt better suited to going fast than comfort. The 1988 Ironman Carbon was lighter, of course, and not as stiff, but probably more able to do a fast 100-miles due to it's comfort level/geometry combination. The 1985 Equipe Centurion is a whole different animal, being pretty much a re-badged Cinelli.

In my experience only:
1981 Semi Pro - touring, centuries, brevets and any other group ride where you want to out-Rivendell the yups and aren't being hectic.
1983 Turbo, 1984 Turbo - same as above, no eyelets, Rivendellers won't notice it, and you can ride in the hectic pace line.
1984 Comp TA - same as the Turbo, in my experience.
1984 Lemans RS - same as the Semi-Pro, but heavier and not quite as smooth. A great workhorse bike. No bling on it.
1985 Ironman - Same as the Comp TA, but with a slightly lighter frame. I couldn't tell the difference.
1986 Ironman - Smooth, light, balanced, seemed to combine the best of the Turbo and the Comp TA/85 Ironman.
1986 Facet - Light, balanced, stiff, only a CF fork away from being a great racer, hard on the butt.
1987 Ironman Expert - One of the best values in bikes I've seen. I've seen and ridden them in road and TT/Tri mode, very very capable.
1988 Expert/Master - No different than the 1987, but if you're buying used, the Master has the tricolor. Lighter wheels, or seemed so.
1988 Ironman Carbon - Light, fast, quick geometry. Not your BB30 stiffness, if that's what you expect. Long-distance? Great.
1989 Expert - the blue/smoke seemed exactly the same as the 88's, the black seemed twitchier. Could be my imagination or the setup.
1989 Master - best all-around bike I've ever had, but truly, any of the 87-89 frames, set up the same, should be the same.

1985 Equipe Centurion - just not part of that genre, in my view. It's a Cinelli, indistinguishable from my SC as far as ride.
If I had to do 100 miles in a pace line that was pushing it but not killing it, I'd take an 87-89 Master with my setup. Within a couple of hours, I'd be ready to go again.
If I had to do 100 miles in a fast pace line or was pushing for time, solo, I'd take the Equipe. I'd suffer more, not sure if I'd be faster, but likely so, as it's a race bike. My butt would hurt for a couple of days.

It sounds a bit like the renewal poster is looking for information to either enforce a value, or simply be accurate. Without knowing the setup, especially the drivetrain including tires/wheels, it's hard to say. Higher end components tend to bring out the better qualites in a good frameset, but the setup is very important. I've seen an Ironman ride totally different with Mavic Ksyrium Elites vs. Bontrager Race vs. DA/Aeroheads vs. 600/Araya OEM's.