I'd probably recommend getting both like you said if the OP's LS is hardcore about Tiamats.
I think for us, we're kind of like your LS is right now - not really looking to do Tiamats. We'll kind of do it if we have a lot of bored people on and it's just sitting there... which almost never happens. If it were like a year ago maybe our tanks would be hardcore enough to do this, but probably not worth taking at this point
For utility purposes, there are lots of things you can't apply a fire resist set to, but you can apply a magic damage down set to just about an HNM. With the way tanking has progressed now, I think a good Magic Damage set is pretty standard.