https://roleplayersrespite.com/5e-advantage-disadvantage

What's Better? Resilience (CON) or War Caster?

There are two answers to this question and it mostly comes down to what you're looking for from each feat. If your character would benefit from any of the utility aspects of War Caster, War Caster is better. Any sort of hybrid/gish character will probably want War Caster.

Some other obvious (and not so obvious) things to consider...

• Will Resilience even out your CON? If Resilience increases your CON modifier, it's probably better than War Caster.
• Do you have abilities or traits that negate low rolls? A Divination Wizard, the Rogue's Reliable Talent, and the Lucky Feat all make rolls with profiency bonuses better.

The better question to ask is...

What's a better way to help Concentration Saving Throws? Adding your proficency bonus or having advantage?

In almost every situation here getting a bonus to your saving throw is better than advantage.

When is Advantage good?

It makes easier checks easier but doesn't make harder checks easier -- advantage is good when you're expected to succeed in the check.

This might seem counter-intuitive but if you have a low chance of making a save, advantage gives you a second chance of success but you're still likely to fail the check. The average roll for a d20 is 10.5, rolling more dice doesn't change the average value.

What does the math say?

The general rule of thumb with advantage is that it is roughly a +5 bonus to your roll but that's not quite accurate. If you're trying to roll a 15 or higher, then the bonus is closer to a +3 -- and this value gets lower as the desired roll gets higher.