It depends on your level (among other things)
tl;dr: Advantage roughly equals a bonus of +5.
Hence, until you're level 13 and your proficiency bonus increases to +5, War Caster will be more useful.
If the ability score increase also raises your CON modifier, you reach the tipping point 4 levels earlier, at level 9.
Let's do a little more math. Assuming you often take less than 20 damage, you will usually have to beat DCs of 10. Assuming you have 14 CON, let's take a look at some anydice.com calculations:
Probability to roll at least 10 with advantage and a +2 modifier: 87.75%
Probability to roll at least 10 without advantage and a +5 modifier (+3 prof): 80%
Probability to roll at least 10 without advantage and a +6 modifier (+4 prof): 85%
Probability to roll at least 10 without advantage and a +7 modifier (+5 prof): 90%
Probability to roll at least 10 without advantage and a +8 modifier (+6 prof): 95%
Clearly, your best option would be getting a +9 modifier, in which case you simply couldn't fail concentration checks with a DC of less than 10.
However, that would require 16 CON and the maximum possible proficiency bonus, so it's unlikely you'll get there before you'll have to suceed on higher DCs than 10.
Anyways, with 14 CON (although that shouldn't matter too much), you need at least a proficiency bonus of +5 to get better results than by choosing War Caster. If your CON score is uneven before you take the feat, it will also increase your CON modifier by one, meaning you only need a +4 proficiency bonus (which is reached at level 9).
Be sure to keep in mind that War Caster provides other benefits - being able to cast (certain) spells as opportunity attacks can be really useful. For this reason, I'd always advise choosing War Caster.
Then again, Resilient benefits all CON saving throws, not just concentration saving throws, so I guess it depends on what you want.
However, if you can spare the ASIs (e.g. because you're a variant human and get one free feat anyways), choosing both can be really powerful.
Probabilities when choosing both feats:
Assuming you spend your first two ASIs at level 4 and 8 on actual ability score increases for your main stat and are playing a variant human, you'll be level 12 by the time you accumulate 2 feats.
This means a proficiency bonus of +4. Assuming you have 14 CON and don't gain a modifier increase from Resilient, you have a 90% chance to roll 13 and higher, and a 50% chance to roll 21 or higher.
Anydice link; select the "at least" view.