Go for relic +1 or AF2 helm?

I'm about to get my relic but my question is, how much of a dps increase for a PLD from a relic to +1? is it worth over the stats of the AF2 helm from a AF1 helm?

Get the weapon, it's a major improvement to threat. If you're worried about replacing your AF1 helm, just replace it with Darklight. You're better off getting the allagan helm later and saving your myth points that you'll spend on armor for the valor legs.

Valor legs should be your first armor purchase because they are superior to the allagan legs. They allow you to do an itemization build that hit caps you while giving you the best defense and well rounded offensive stats possible using other item combinations.

Also, when I was checking some min/max gear combinations, it was almost always better to get the allagan helm over the valor one. It's best not to waste your myth on the valor helm.

Now if you want to compare getting valor legs vs getting relic +1 weapon that's a whole different discussion. I'd personally go for relic+1 weapon then pick up valor legs rather than vice versa. The damage upgrade is very noticeable when it comes to threat generation and will help you a lot in Turn 4 when you need to pull snap aggro on Dreadnaughts.

It's really up to you though. Comes down mostly to personal preference.

Allagan helm drops on turn 5 for PLD. Something that no one has legit cleared. It is also the best total point per point of mythology. Allagan chest armor drops on turn ONE, and anyone can clear that.

Finally, threat is NOT that big of a difference.