Is this a good idea for strength? I would still plan on insulating the attic to around R55. What would be the price difference between a traditional stick vs a steel joist system? More or less?

You say "joists" but do you mean trusses? If so, I like the idea, but I priced out both. Eleven thousand for wood trusses and twenty-seven for steel.

I'm not sure how to compare both options. I use open web steel joists with a concrete slab on metal decking. The only similarity to a wood roof solution is that it's a roof. Everything else is totally different. Concrete slab vs. plywood on wood joists, steel joists welded to concrete embeds vs. wood joists nailed to wall, different insulation, different roof thickness...
FWIW, the steel joists & metal deck for my roof (flat, 750sf) were ~$3000.

I would get a quote for a concrete roof also. quaddeck, insuldeck, litedeck, builddeck, etc can be the form (and at least some of the insulation, just like the ICF walls) and you pour the concrete on top. It makes for a stronger overall house and the concrete connection from the walls to the roof gives you an airtight barrier. The budgetary pricing was ~$5.33/sf for materials (deck, concrete and rebar) not including labor.

One thing to check on your pricing is the extra depth on the steel/wood trusses at the walls if you want a uniform depth of insulation in the attic. I don't know how many inches R-55 insulation is, but it is probably more than a foot of insulation and thus a foot for the trusses.

You might also want to check out the thermal bridging differences between wood and steel. In general, steel trusses are going to conduct much more than wood. So if you have the typical cold/hot attic, then that temperature is going to be telegraphed down through the steel or wood truss.

Personally, I am leaning towards a concrete roof with 3lb closed cell spray foam on top and a cool roof membrane on top of the spray foam. This shields from a thermal bridge from concrete roof to concrete walls. 3" of spray foam on top and the deck form insulation put's you at your R55. But you also get a first rate air barrier seamlessly from wall to roof to wall and you get continuous insulation over the same stretch.

You might also think about whether you want structural wood with ICF. The wood can rot, mold, be eaten by bugs, etc. If you see the advantage of ICF for the walls, a lot of those same advantages follow with the ICF decking for the roof.