Shadow Walkers have powerful yet short buffs for Eva and none for Void.
Having both stats may seem nice, but I fear that mixing Eva and Void will end being less effective than a focused build, so between the two I would go for Eva, because of the buffs.
Void will grow more and more interesting as attacks will be more and more powerful, but again SW has no natural 'affinity' for Void.
My 2 cents.
It's really difficult to find checks and balances in the way the gear was designed.
For example, Having EVA on Leg Guards doesn't prevent you from also having VOID. It does take away around 99 HP.
Shoes Obviously Favor EVA, but let you decide between HP and VOID.
Tops Favor Void, But Allow for EVA or SP (most choose EVA).
Helmets Favor VOID, but allow for the highest bonus to CH-EVA as well. This is probably the only situation where you have to CHOOSE, with the exception of accessories.
Just choose to have both on most of your gear, and focus your accessories towards the one you prefer to have.