I know how Warcaster works. I've run simulations generating two sets of 10 billion d20 rolls and the average is 13.7. That is 3.2 points better than the average of a d20 roll (10.5).
At level 5 your proficiency bonus is +3. That's just slightly worse than 3.2. But it's better if the +1 CON bumps you up to a total of +4 compared to what you had before. In that case your average roll would be 14.5. At level 9 your average roll is 15.5 compared to 13.7 from Warcaster. (Note: I didn't add in any CON bonuses because I don't know what people are going to have. My one assumption for this paragraph is that whatever your CON was, it was an odd number so Resilient bumped it up to an even number)
Average roll doesn't matter, though, since what you're after is not being below a certain point. So what are the odds of failing a certain DC? Well, let's compare those numbers.
# = number you need to roll higher than to succeed, without the extra Resilient modifiers.
DC = What you need to beat, going up to 25 to account for Resilient, but a 20 will always succeed.
WCFR = Warcaster Failure rate in percent.
RFR = Resilient Failure Rate, and for this, going best case scenario, of +1 extra bonus from con and +4 prof. In percent.
And since those are failure rates, a lower percent is better.
DC - BFR - WCFR - RFR
25 - 95% - 90.25% - 95%
24 - 95% - 90.25% - 90%
23 - 95% - 90.25% - 85%
22 - 95% - 90.25% - 80%
21 - 95% - 90.25% - 75%
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20 - 95% - 90.25% - 70%
19 - 90% - 81.00% - 65%
18 - 85% - 72.25% - 60%
17 - 80% - 64.00% - 55%
16 - 75% - 56.25% - 50%
15 - 70% - 49.00% - 45%
14 - 65% - 42.25% - 40%
13 - 60% - 36.00% - 35%
12 - 55% - 30.25% - 30%
10 - 50% - 25.00% - 25%
09 - 45% - 20.25% - 20%
08 - 40% - 16.00% - 15%
07 - 35% - 12.25% - 10%
06 - 30% - 9.00% - 5%
05 - 25% - 6.25% - 5%
04 - 20% - 4.00% - 5%
03 - 15% - 2.25% - 5%
02 - 10% - 1.00% - 5%
01 - 05% - 0.25% - 5%
So from that, assuming I didn't screw up the math, you can see where each has its advantages. Overall, if you typically get hit for 20 or less damage, and have plenty of saving throw bonuses from gear, warcaster wins out. But if you are routinely getting hit harder, have low saving throw bonuses from whatever, and you're con is pretty low, Resilient wins out (if only barely for certain bits, with a tie at 10) until you reach a point where you're screwed regardless. And for much of it, you won't notice the difference either way.
And the best, if you want to get serious and do something like a druid who strolls through his own thorn patches, is to have both. If you're MC'ing into Barb, Fighter, or Sorc, make one of those your initial class, at least on respec when the time comes. If your gear can get you either advantage or proficiency, get the one that you are lacking. If you're a halfling, go Resilient.
