Question about which gem to use as my 6th link in sunder

I'm trying to 6 link a chest armor and whenever I eventually get it done I would like to know which gem I should put in the last slot. I'm playing pretty much purely phys sunder with hatred and herald of ash for some extra dps. I'm running Sunder > Multistrike > Maim > Melee Physical Damage > Ruthless right now. Should I change any of these that I already have into something different and what should I use for my 6th? I was thinking about faster attacks or maybe conc effect but I'm not sure. Thanks for any tips