Spear of the Magus?

I also personally believe that the spear of magus is a great item. However we have to also count what we lose. As we gain the extra health and magical penetration, we lose the chance of moving faster with the boots. With mobility, it might not be necessary to use the spear. I guess its with experience? As you get use to dodging skill shots, mobility will definitely be more helpful in the long run too. Both boots and spear are great items, and I personally think it depends on the play style and the experience a person has. Another considerable minority item that could be a starter is the pendant. Although it is not a great, most really experience players sometimes use it ( I think).

To come to the decision between spear and void stone:

I'd say it depends on your role -

If you wanna be carry, get the spear cause health
boosts your resistance against both phys and mag dmg.

If you wanna play (semi)support (what is an option for example Ra)
void stone would mostly be better, because your team
benefits from the magic pen aura.