The advantage of glyph seals over spell storing ammunition are quite a few
It can store AoE spells, and if you read it by RAW, lesser seals store up to level 2 and greater seals store 5 and above. Access to 9th level spells on your bullets is fun. Even if you read it RAI and treat it as 5 and below, it's still more versatile than spell storing ammunition.
Plus there's that whole debate about whether spell storing ammunition is legal wheras glyph seals are already legal and the debate is just how powerful they are.