Seth has combo attack down, which is one of the more valuable debuffs. Wind is already strong against overdrives due to Gaia, but is still vulnerable to random triple attacks. Seth fixes that without gimping the team's damage. In addition, combo attack down is always useful, but dispel is only useful sometimes. When needed, it can be equipped as an EX skill.
Combining Cthulhu with Sol in an off-element team is a bad idea. Going from -40% to -50% DEF down and 4 to 3 same-element kamihime with eidolon buffs held constant, damage is slightly reduced. And that's assuming Cthulhu's debuff even lands.
If you burn too many Miracle tickets on "safe" choices that can fit any element, the result will be 6 mediocre teams. At some point, you have to commit, and my advice is to commit on the first Miracle ticket. This assumes that the average player who would spend on Miracle tickets has a very low chance (<10%) of later obtaining a +100% eidolon--and I haven't seen any evidence this is incorrect.