In your same boat, been debating so long that I am now up to 10 Mystic Rings and by the time I make a choice, I may have enough to just do both and be done with it.
Save them for December
Main question is how much emblems you have for both heroes? JF is amazing if you have emblems for him. Without emblems I wouldn’t prefer him.
Since it’s not been mentioned (I know thread is old)… One good reason to pick JF is synergy with Killhare.
He’s an okay defender (but defence shouldn’t be a priority until very late on), but Tyr is generally better on defence anyway.
JF is pretty situational, where Tyr is a one man army… But JF’s party trick is the synergy with the overpowered Bunny!
So i’ve had both for months now and tried both on both war D and raid D. I can definitely say which one you use is situational. JF was strongly better on raid D with both Heim and them Frigg as a tank. War D, Tyr really shines on a wing during field aid, otherwise JF is better in the other legacy formats. JF is useless during equalizer, so Tyr would be better here too.