are you talking about decoding engrams? if so, it doesnt matter what you're wearing anymore....the game automatically finds out your highest level for a character by searching your account, and taking a combo of the highest weapons (no matter the character) and the highest gear (character specific).....so whatever you have (whether it's in the vault, just on your character not being worn, etc), the game will find it and calculate your level for engrams...
if you talking about just playing/doing shyt, either keep both and use them depending on what level you need to be; or infuse the blue into the exotic....that's what i did for my warlock helm....that shyt went from 105 to 266 lol....
speaking of which: just remember when infusing exotics, they go higher than what you are using to infuse into it up to 5 points i think...so that's a way to increase your level easily...