Originally Posted by saberlance

The gacha equipment comes with a random +0 to +10 modifier. You are correct that we can't change the modifier, only level. So for long term efficiency only upgrade +10 drops (and special equipment from events or character quests).

Actually, you should be upgrading non-10s, with the assumption they'll be used as fodder for +10s or special equipment. Just don't upgrade fodder more than once.

A level 9 fodder is worth double the xp of a level 1. For bronze equipment like a Defense Bracelet, feeding one lv1 into another will create a lv10(lv11 for a critical success). Using that lv10 as fodder will give more xp than using two lv1s for the same total cost. You can max a Defense Bracelet or Defense Necklace by feeding it 7 lv10s + 1 Lv11 for a total cost of 16 pieces of fodder and 2880 coins(+1 fodder and 180 coins if you didn't get any critical success and use 8 lv10s).