Usually you infuse the purple armor/wep that has good perks. I use my rare(blue gear) onto my legendary(purple gear) to make the light level higher. Weapons like hung jury, hawksaw and the 1000 year stare you want to upgrade to highest level possible because usually those weapons are good stock and with some perks they are even better. Armour wise I like to use pulse rifles and snipers so I want legendary gloves that have faster pulse reload, chest that has pulse or sniper ammo and same with the boots. I like intellect and discipline on all my armor. If I can't get discipline with the intellect I go with the intellect and strength.

its like upgrading your weapon to a higher light. so i would suggest upgrade the weapon with a better roll/perks