Circle of the Shepherd Druid Guide to Fuzzy Fury

[Nhym]

Advantage can equate a +5, but it does depend also on the DC to reach and your innate bonus.

Until level >8-9, the occurence of you actually having to save against anything other than a DC 10 should represent something like 5%, at most 10% if you're unlucky, of all rolls of your all life.

If you pick it at level 4, you're one level away of proficiency upgrade, right when you get the precious Conjure Animals. And you can even CON score, so it's an immediate +1.
At level 5, you already have a +4 bonus to the save compared to the Warcaster (+1 from evening CON, +3 from prof).

You just said it yourself: At level 5 Warcaster equated to an effective +5 on the majority of concentration checks while Res [Con] is a +4 Bonus. That right there is ALL I am saying, that +5 is higher than +4. And as my source indicates, "If the ability score increase also raises your CON modifier, you reach the tipping point 4 levels earlier, at level 9." Which is one level past your second ASI at level 8 where I would have Res [Con] grabbed.

So put simply, when raising an uneven Con modifier, Res [Con] offers an effective +4 to concentration checks until level 9 when it hits an effective +5. Warcaster offers an effective +5 always. Therefore when solely concerning concentration checks, Warcaster is better at lower levels.

Remember, I don't suggest against taking Res [Con], far from it, and in the guide they are exactly the same color. I am simply advocating for taking Warcaster at earlier levels.

Since you're so adamant on having a good concentration, starting with 15 CON (or maybe even 17) should be one of your top strategies, possibly even more than having high WIS if you really rely solely on your summons (and Hill Dwarf should be top pick then with +2 CON and +1 WIS IIRC ;)).

Yes, and Hill Dwarf IS a top pick.