[quote=Revans Exile]
Reach Armor nothing more than the Mark V seen in Halo and Halo 2 before upgrading to the Mark VI. The only difference is you get to use different variations of it.
[/quote]

Halo 2 used Mk VI as well, Combat Evolved and Reach were the only ones to use Mk V. And in general, most variations had a set for both V and VI, but a set was made for the armor being used at the time.
ex.
[hider=Reach EOD][img=http://i49.tinypic.com/w2oln4.png][/hider]
[hider=Halo 3 EOD][img=http://img4.wikia.nocookie.net/__cb20090722051653/halo/images/2/29/EODArmor.png][/hider]