Each has an advantage over the other. IMHO one is not better than the other. Each is simply different. Diciding which differences will best suit your needs will determine which is best for you and your needs. Not which is generally better than the other.
TA11 has longer, more forgiving eye relief. Therefore, head position is not *as critical* on the TA11 as it will be on the TA31.
TA31 is smaller and lighter with .5x more magnification. It also has a wider field of view. There was also something about how the reticle seems a little bigger in the TA31 than on the TA11 because of its slightly higher magnification. I cant recall exactly what it was, I'll have to look through my friends TA11 again and Im sure it will come back to me. I think its like a variable power scope whose reticle will get larger and larger as you increase the magnification. If this is in fact what it is, the larger reticle will likely be easier and faster to see.