Hi,

all the common species of larger rush (inflexus, effusus and conglomeratus) vary quite a lot in the compact-ness of their flowering head. Hard rushes tend to have the more diffuse inflorescence seen in your second plant but it may be much denser. if the morphology of the stem says its hard rush then that's probably what it is. The generally rather narrow, very ridged very greyish stem is pretty characteristic.

There is a lot of hybridization amongst these rushes, the hybrids not always sterile. They are generally intermediate in many respects but are not easy to determine and stem anatomy may be the most decisive way to name things. There are useful tips in the BSBI's Plant Crib. Both parents tend to be present but not always. However I reckon you've just got different forms of inflexus there

Best wishes

Fred