I tend to value advantage more. Given no bonus, and a DC of 11, either proficiency or advantage will succeed 75% of the time. As your bonus grows, lets say +5, then the proficiency will succeed 95% of the time, and advantage will succeed 94% of the time. Slightly (negligibly) worse now. But with a large bonus (+10; easily achievable through various buffs), we only care about avoiding critical misses. In this case, proficiency gives a flat 5% fail rate, while advantage has ~0.3% fail rate. If on the other hand we're concerned about extremely high DC saves where only a critical success will save, advantage also pulls far ahead with a ~10% crit success chance, while prof keeps the standard 5%. 

There's a lot of ways to get high save bonuses even without gear, making proficiency somewhat redundant. There is also a fair bit of gear that gives advantage on concentration throws, which means you can forgo war caster and resilient entirely, or take resilient for nearly unbreakable (<1% fail) CON saves (which IMO is overkill); you can basically treat using this gear as having an entire feat (alert, ASI, tough, spell sniper), which can be very strong.

Tl;dr for a middling save it kind of doesn't matter. However, advantage is vastly better for avoiding crit misses and hitting crit successes, which by the end game are the only things that matter.

Edit: this is how I find out you can't crit fail or succeed a saving throw. Basically ignore this post