Can I replace the engineer's I-Beam recommendation with a different one?
I think I figured it out.  I was thrown off by the Elastic Section Modulus.

Anyway, the deflection is calculated using the following parameters:

W = load

L = length

E = Young's Modulus

I = moment of inertia

And the equation for max deflection is: 



I'm trying to find a beam with the same maxdef or smaller.  I can ignore W and L because they are the same regardless of I-beam.  Young's Modulus E is a property of the steel used, so it is also the same regardless of I-beam and can be ignored (this is what I misunderstood).  This means, the only variable I have to worry about is I.  I can see in the equation that I is part of the denominator, so if I increases, deflection decreases.  So I just need a W6 beam with a larger I than what the W8x15 beam has.

Final answer:  The only W6 beam with a greater I than the W8x15 beam is the W6x25,