Both are great. My main commando has resilience, but it's because I got him from the beginning and I want him to survive through the whole game. My other commando has killer instinct and he can OHK almost anything but a Sectopod.
With resilience there's a reasonable limit to how much damage your guy can take in 1 round, which lets you safely dash into the fray without fear of dying. When a defensive option removes a great deal of luck from engagements and lets you play even more aggressively it's a clear winner.