The Oldblood is over-armored when mounted, but the nice thing about it is that he remains at 1+ AS even if the Carnosaur is killed. But the main consideration is points, which are pretty tight. As you note, light armor + talisman ends up costing more than the armor, plus there is the cost of whatever item I would try to fit in the magic armor slot. I could drop some of the command models, and I might do that in other circumstances, but too many of the scenarios rely on having "scoring units" containing at least one command model.
That's a good point about retaining the 1+ save, I hadn't thought of that. I've got to say, I really like your list. It looks like it would be fun as hell to play with. Good luck mate.