Personally I think that both are great sources of tests, and there are not a lot of worthwhile programs that offer accurate mock tests. The DAT achiever is definitely harder than the real thing, but is a great study tool. Topscore is the most accurate format to the real thing. To answer the question of when you're ready, I don't know. Pick a reasonable date, and stick with it. Personally, I got sick of studying. 😀 Hope this helped.