both achiever is harder than topscore. try topscore first then go achiever if you have time that is. if not just by achiever and be prepared for a challenge.
I used both programs for my studying and the DAT achiever was WAY harder than the actual DAT. I was scoring around 19-20 on TopScore Pro and around 16-17 with DAT achiever. On the actual DAT (just wrote it two days ago) I scored a 20. I would say TopScore replicates the DAT more closely than achiever. The only thing Achiever did for me was stress me out!