spectra premium heater core issue ? / anyone recommend a good heater core ?

Aluminum or brass won't make much difference to felt heat. The aluminum will tolerate higher pressure though.

The thermal conductivity of the two materials is nearly the same. Aluminum has a higher specific heat capacity, roughly double that of brass. All that means is it will take a little longer to warm up, but it won't affect final temperature.

If you have an alum rad, the core should be too since most alum rads use higher pressure caps.