I just did Math Destroyer and haven't looked at QR destroyer. I've taken 6 practice tests and compared to the 1st test I took, I feel so much comfortable now. I think QR and GC are my best 2 subjects right now. On Math Destroyer in the beginning I was getting 23-24/40. Now I am getting 32-34/40. This is my first time taking these practice tests through destroyer too.
If you have the time, then definitely do both. It'll help a lot because "practice makes perfect"