I used both. My TopScore scores were somewhere between 21-22 and I ended up with 22 on the real test. So, I think TopScore is a good representation of the real exam. I used achiever and it DESTROYED my confidence two week before the exam. My scores dropped from 22 down to 18-19. So, I'd say Achiever is much harder than the real DAT would ever get. You can still use it if you want some extra practice though.
Good Luck👍