they do different things. One is lightning and forces you to play with -200% light res and work around that. One is fire and all you need is to stack fire res to overcap up to 200%.
Ignoring what ability you plan on using the ring is way better just from how easy it is to use and fit into most builds. The ring is also any fire dmg you deal, where the chest is "nearby enemies" Which means if you're off screening things (or your merc is being dumb), the chest is doing nothing. The ring is your dmg so it is always applied.
I guess the chest is cool because you can slap it on your merc and instantly get value from it but I'm not sure how much it would die in maps. My merc is pretty glass cannon gear wise and hasn't died once but -200% is 12 times lightning dmg taken compared to 75% res lol
Ya I got dorys last night and was doing some insane DMG lol and perma freezing bosses for the second or 2 they were alive which was cool..then I did some Polaris invitation and got one shot by a meteor lol prob gonna reroll that ..the fire might be better but the freeze is insane with the lightning / replica heatshiver etc