I've been toying with a pair of Treks, a 412 and a 612. The 412 is Ishiwata 022 steel, 42/52 crank and a 6 speed freewheel on relatively fat tires. The 612 is 531, 50/45/28 crank and a 7 speed with similar range of gears on nice think rims and tires. It seems to have better hubs and bearings and a few other nicer details. The 612 is also a prettier color. That seems to help.
The 612 is faster. I've tried switching wheels between them. It helped narrow the gap but it didn't close the gap. I haven't tried switching cranks yet.
Everyone likes the Ishiwata steel and says it's comparable but I go faster on the 531. Don't really know why.