azathioprine, imuran and 6-mp...which is better?

I fear the azathioprine is not working for me.  I wonder if imuran or 6-mp would be possible alternatives or if aza is not working for me then the others wont either?  Are we on what we are on b/c of the way our bodies metabolize them?

When you first start taking either AZA or 6mp, you take the Prometheus test. This tests for certain levels in your blood and the doctor determines which one you should do better on. If for some reason you are unable to tolerate one, then the doc can change it over to the other unless there is a certain toxicity. AZA breaks down into 6mp in the liver so 6mp is the purer form so the liver doesn't have to work as hard to break it down. Some people need that break down in order to metabolize it correctly while others don't. I have been on 6mp now for 1 1/2 years and I first had the Prometheus test to determine which one would be better for me and it seems my doc put me on the right one. But I had to have dose increases because I wasn't metabolizing it properly.