@Talismina not bad but you're missing a few things in there- namely that the DC of 10 is the minimum, and also, a bit of context on what the effect is of your overall bonuses. Here's the odds of a failure if against a specific target DC with a specific save bonus normally:

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And here is the odds of failure if you have advantage:

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This of course continues on infinitely in both directions, but the failure chance never gets lower than 5% (or .25% with advantage) or higher than 95% (or 90.25% with advantage) and thus it's just a continuation of the same trends. Getting Resilient moves you right (or up, it's the same thing) 2 squares at level 4 since it's +2 to your save bonus, or up to 4 squares at level 12. Getting advantage moves you to the same square on the advantage table. As with the original chart these are failure rates, or the chance you'll Lose concentration, so the lower the better.

I've color coded it - Red, Advantage (and thus Warcaster) is always better than even a +4 save bonus. Yellow, and a +4 bonus is better but a +2 is not, so Resilient is better at higher levels while Warcaster is better at lower levels. Green, and Resilient is always better, even at only a +2. A few things to note - the yellow edges, Resilient is *barely* better with a +4. 5% instead of 6.25%, or 55% instead of 56.25%, or 90% instead of 90.25%. Also, the very best case for Resilient, where you had a DC 20 check (meaning you took 40 damage) and have a 75% chance of failure instead of 90.25% chance of failure, you still have a 75% chance of failure. It's far more likely than not you're losing concentration there.

The reason the bonuses are stretching so high is because you can realistically get +10 or more to your saves - Bless (especially with the Bless staff), the Robes of Supreme Defense, the Paladin Aura, the Barbarian ability, shields, rings, and proficiency, all of them add to your overall bonus. Just with an 18 Cha Paladin, the Con amulet, proficiency, and the robes with a 20 in your casting stat, and you're already at +19.

This, combined with the reality of the situations where Resilient is significantly better, where you're taking 40+ damage in a hit and are more likely than not losing concentration anyway, is why I prefer Warcaster generally. It's also the case that I don't typically think it's worth taking both, which is why I don't have Resilient higher, but you certainly could, if you absolutely never wanted to fail a concentration check.