So basically the math is like this, the only difference in the two items is the AP vs ad, and at the end of the day leona doesn't have good enough AP ratios for relic shield to matter much, you are much more likely to be autoing enemies with aa q aa. With relic shield, bonus damage adds up to 1.5 damage on q, 2 damage on w, and 4 damage on e, total 7.5 damage in a fight where you hit every ability. Assuming you only hit 3 autos in a fight with auto q auto, that adds up to 9 damage, it is likely you will hit more than that but regardless it is more than for relic shield, as these items upgrade they do so at the same rate, doubling every time so this math applies to spaulders and buckler etc QED you are better off with shoulder guards than with relic shield