I assume you are referring to
this wheel and
this one, or the eight inch equivalents. We have only ever used the plastic ones, which have worked fine for us. The main advantage of the plastic wheels is that they have space for two bearings, which is nice for stability in dead axle applications. If you are using live axles, this feature is not as necessary, as you would probably have some type of hub providing further stability as well as mating to the axle. The aluminum omniwheels, on the other hand, have the advantage of being more repairable, as if they are hit on the side and bent, they can simply be bent back into place, where if the plastic wheels break, they will likely snap, and require a replacement wheel. So it really depends on which is more important to your design. If there is absolutely no way anything will ever hit your wheels, the plastic wheels are a good choice, but if they are out in the open, you may want to look into ways to get more stability out of the aluminum omniwheels, such as doubling them up.
I'm a bit confused on what you mean by your question about wheel angles. Do you mean which of these two overhead views would work better?
Our team has never done this type of drive, but I believe that if you work the vectors, you get more power out of four wheels pushing against eachother at angles, as seen in the top drawing, compared to 2 wheels with full power, as driving forward with the bottom drawing would do. You would probably want your maximum power to occur in common directions like forward and sideways, rather than 45 degree angles.
Don't know if any CAD models exist of the Plastic omniwheels.