I also do tai chi, have been for some time. I have not found the actual movements to be beneficial for back pain. However what it teaches you to do can be helpful.

After a year and a half of doing tai chi i finally found how to make myself relax. Then my body seems to move freely with little or no tension and not a twinge of back pain.

stress causes muscle tension which causes back pain.

I have not tried pilates though.

Duke