Depends if you want the unit or not. If Maribel is going to be a great unit to your team then yes you are better off using for CR and SU. But if you don't need her you can always use them for xp fodder. It all comes down to what you want, as I don't have a need for her as I have two stronger Heavy Armors but I still use my duplicates to Cost reduce and Skill up and have some left over for other things.
The duplicate units wont offer you anywhere near as much xp as Plat Armors they are just a bit better than using bronze units for upgrades so I find it better to add to your main event unit. Event units uses are limited as they aren't treated like normal units of that tier, you wont get much Rainbow Crystals when discharging them and during double or triple combined xp weeks for gold tier and above units even units don't get this buff so no point keeping them for that.