If it is for a HW robot I would go for as strong as you can.

On Cherub the lifting forks were 8mm thick, High grade steel as used on nuclear submarine hulls ................It bent! the second set were made of 10mm steel the foundry didn't tell me what the steel type was but I couldn't drill it with anything I had. and it only bent when heated up first then put in a large hydraulic press.

They didn't bend in combat but the 30mm solid steel shaft they were connected to did!