Get the Myth helm for sole reason that Allagan helm drops from T5. Unless you have a group that you know can reliable clear T5, get the Myth helm eventually for sure. Any minor differences in 2ndary stats is far from making or breaking you, but the differences in STR/VIT/DEF? from upgrading to ilv 90 can make a difference. If you have a reliable T5 group and only want "tanking" stats, then you can ignore the Myth helm. T1-4 can be fairly easily farm with PF groups, T5 generally cannot.