you can use whatever looks cool. i like the veteran merc armor with the vest, the bandoliers across the chest, and the shemagh tied around your neck. having DR would technically be better since you can still get hit but you look way cool with the beret, shades, and the merc veteran armor :D
I just use more lighter armors in the game simply because of the weight reduction. But as previous poster stated I;m going to be upgrading to the Recon armor shortly. Anything that has a plus agility is nice. plus criticals etc is also nice. Same for sneak so you can move in a closer to get those good VATS long range headshots etc. In FO3 I never got into being a "tank" personally just because I like to sneak around. Most of those heavier armors have bonuses plus negatives.
Also to off set some to the damage protection loss from light to medium armor you can add the implant that adds DP, take tough skin perk etc. If the enemy closes on you there is a perk that decreases melee damage etc if you choose to use these perks.