Liked the suggestion below yours of ribbing the aluminum plate as well. I'm sure the grade of aluminum matters to and all.

So by 'hold heat better' I imagine you mean like a pizza stone or a thick frying pan: it takes a while to heat up, but changes in conditions (things like drafts or just putting the pizza on it) doesn't change it's temperature much? If so, this is related to the material's heat capacity, and the quantity of the material (relative to it's surface area). Higher is better:

Brass: 380 J/kg/K
Aluminium: 900 J/kg/K

Note that the density is very different:

Brass: 8500 kg/m^3
Aluminium: 2700 kg/m^3

Combining the above, if you assume you're using the same thickness of plate, then the brass does actually hold more heat (about 1.3x). It will also weigh more than 3x as much.

For 'evenly', you want high conductivity as Tomek suggests: the unevenness comes as there is heat being put into the plate in one place, but it's leaving at different rates from other parts of the plate. If the conductivity is high, the heat moves through the plate without much of a temperature difference.

Permanent warping is an issue with the processing of the material - ideally you'd run the plate (which comes out of a rolling machine with all the commensurate internal stresses) through many fairly extreme thermal cycles until it settled down, then machine it parallel again. This would be an issue with either material, and depend on the way it's manufactured (e.g. cast aluminium might not do this at all - not sure here, too much to look up at once!).

Warping due to a temperature difference between the top and bottom surfaces of the plate is improved with higher conductivity (keeps the internal temp diff down), and lower coefficient of thermal expansion (how much the material expands per degree of temperature change). Also, a thicker plate means that for the same difference in linear length at the outer surfaces, the bend radius would be larger (i.e. not as tight, less bending!). Brass and aluminium are about the same in expansion anyway:

Aluminium: 22.2 x 10^-6 m/m/K
Brass: 18.7 x 10^-6 m/m/K

Then there's cost. Brass is mostly copper, and copper is expensive. You'd be better off using a thicker aluminium plate than using brass, as this gives you all the advantages with none of the disadvantages!

Why is aluminum the bed material of choice?

excellent conductivity
excellent strength to weight ratio
excellent strength to cost ratio. 
easily machined and formed.

Think all you want but learn sound engineering practice and when you ask for advise listen to it.
Brass would give a nice Steam punk look.  but more expensive etc.
Tin

Why do you think it would work better?

Aluminum: 237 W·m−1·K−1 (thermal conductivity.)
Brass : 109 W*m -1*K-1 (thermal conductivity.)

You want high conductivity to spread the temperature from the heater.

You also have to consider stiffness, and coefficient of thermal expansion, and typical stock availability (types, etc) to know whether a material works better or worse for the bed material

As far as I'm aware aluminum is the easy easy choice because of
(1) 2nd in class conductivity (pretty great.)
(2) super cheap
(3) quite stiff/weight.

Typically you see brass in high-friction situations, or where you need a softer metal (perhaps a metal gasket, idk.)