+1 Weapon seems to really just be about enmity and damage. If you're always doing speed-runs and dungeons, then having +1 is very useful for faster clears, and to hold the monster for just a little longer when everyone else goes all out.

I'm up to turn 4 in Coil, and while holding enmity is starting to get difficult, I'm finding accuracy to be a more important issue for me right now.

I personally went Relic armour instead of +1, because my mind keeps telling me that i70->i90 is better than i80->i90. Damage might be better on +1 but I've wiped from dying a lot more than wiping because I lost enmity.

In the end, either is a good option. Eventually you'll be getting all i90 anyway so the order in which you pick them isn't a big deal in the long run (depending on how you factor in Coil drops)