Does coffee inhibit the absorption of iron? The answer appears to be yes (see links and references below). So does tea. Note that this does not have to do with caffeine, but rather polyphenols, phytates found in them. So, my answer would be that both regular coffee and decaf coffee inhibit iron, but I don't know whether one does so more than the other. Also, if you add milk to your coffee, that may also have some effect, because calcium inhibits iron absorption.
It also depends on whether it's heme iron (mostly from animals) or non-heme iron (from plant sources, iron fortified foods). This matters if you eat primarily a vegetarian diet, because the recommended intake for non-heme iron is 1.8 times, compared to those who eat meat . NIH's Iron
Dietary Supplement Fact Sheet compiled a great literature review on iron intake. See also CDC.
Hurrell RF, Reddy M, Cook JD. Inhibition of non-haem iron absorption in man by polyphenolic-containing beverages. Br J Nutr. 1999;81:289–95.
Layrisse M1, García-Casal MN, Solano L, Barón MA, Arguello F, Llovera D, Ramírez J, Leets I, Tropper E. Iron bioavailability in humans from breakfasts enriched with iron bis-glycine chelate, phytates and polyphenols. J Nutr. 2000 Sep;130(9):2195-9.
Zijp IM1, Korver O, Tijburg LB. Effect of tea and other dietary factors on iron absorption. Crit Rev Food Sci Nutr. 2000 Sep;40(5):371-98.