I guess I'll take up for the 31, here's my thoughts as an owner of multiple examples of 33/31/11s. I love TA-11s, they have a very forgiving eyebox/eye relief and make picking up a sight picture very fast. Yet when I think of the 31, it is lighter, smaller, (very slightly) higher magnification, better FOV (though to be fair it also takes up a proportionate amount of visual space so this is largely moot, see above pic).
If you shoot NTCH, I'd consider the eye relief moot as the 31 is perfect for this. If you don't, or are shooting a larger rifle (my Scar 17 uses a 11 to avoid scope-eye), I'd recommend the 11. There is no 'best', just best for you. Having said that, for NTCH AR-15s, the positives of the 31 far outweigh the lesser eye relief *for me*, and I say that as a owner/fan of both as well as the 33.