There are three different types of intakes:

Closed Box (ie. Volant, Outlaw)
Open Element (ie. AEM, K&N FIPK)
Drop In filter (ie. K&N)

When looking at manufacturers claims, one shouldn't just look at the HP numbers:

You need to look at the highest average airflow which is directly related to torque (subsequently HP).

Manifold pressure (MAP) is a measurement of restriction, the higher the number the less restriction, so this is another big point.

IAT (Intake Air Temp) recovery is another big factor in narrowing down a good intake.

Closed box intakes do better at lower speeds as there won't be as much of a heat soak issue. The open elements use metal shields which absorbs heat (heat soak).

The open element intakes drop temperatures a lot quicker at higher speeds which in turn creates denser air. The denser air allows for more airflow, which then of course makes more power.

This all being said, intakes (as well as exhausts) are a matter of personal preference, just taking off the stock tube takes off a lot of restriction. Realistically, the differences between intakes (ie. open vs. open and closed vs. closed) will be minimal. One thing I do encourage anyone who purchases an aftermarket unit to do, is check the fitment every couple hundred miles, these things find a way to wiggle loose and can cause a drop in performance as well as MPG.