Ideally you want both i90 rings. If you ever need a smidgen more VIT (pretty much only for Turn 5 if you're missing a lot of left side i90), melded Astral Ring should replace one of them (doesn't really matter which but I'd keep the Hero ring on). Personally I cleared turn5 today with a melded HQ Tourmaline Ring paired with my Allagan healing ring, so if your skill and other gear is up to snuff it doesn't matter that much. Even for Turn 5.