Depends on the rulebook.
In basic 8th you get fighting in an additional rank when not charging, but only when fighting in the front.
This makes spears a better offensive, but worse defensive option. Funnily enough for the longest time I assumed that Spears also required two hands to use, making them a much worse option.
In 9th Age they've got a couple of additional buffs, namely armour piercing and quasi-killing blow against cav and similar big things.
In 9th Edition, they grant an initiative bonus when charged in the front. This by itself is unimpressive, but considering that supporting attacks got removed, it's damn important.
So yeah, Spears are a more offensive choice and better if you've got the rank and file to support the additional attacks. Shields are defensive and better with smaller Saurus units.