Depends on the person. DMPS is better in that it doesn't aggravate candida. Some people can't tolerate DMSA but others find DMSA suits them more.
Unless you have an intolerance to either of them, to reduce any possible side-effects from chelation you have to start on LOW doses and make sure you are taking the right supplements alongside them. Janey did you get some adrenal and thyroid supps?