not counting INT at first but
Grio + Enki = 228 skill + 14 Macc base + 0~30 augment + 10 from Enki = 242~272 Macc
Excal + Shield = 255 + 38 from shield = 293 Magic Acc
so just from magic acc alone. Excal has potentially 21~51 Magic Acc more.
Grio has 19INT base and shield has 3 more than Enki strap. so without Augments the Grio set has 16 more INT with the chance of roughly 15 more INT from augments so roughly 30 more INT.
so based on the dINT that potential 30 INT could be worth 15 magic acc or 30 magic acc.
So it's very situation based on augments and the dINT.
but looking at the numbers it looks like the excal combo for magic acc would be the easier/more consistent combo for straight up magic acc. but if you have amazing augments and the dint lines up in your favour then it would be the Grio set.
Or so i believe. If i did something wrong here please correct me.