At 45kVA, not much difference. Aluminum is less conductive than copper, so the windings end up bigger with Al, requiring more steel as well, resulting in a physically larger transformer for the same capacity. But 45kVA is still relatively "small" in the transformer world, so the difference in size is not going to be dramatic if anything at all because manufacturers tend to use common "case" sizes regardless of what the core looks like inside. Cu expands and contracts less with heat than Al, so you need to periodically check connections on Al transformers more often because the Al will suffer from "creep", meaning the Al will flow away from pressure points like mechanical lugs and the primary windings tend to become brittle with age, resulting in a higher failure rate on older transformers if there is a fault somewhere on the primary side. Again though, at 45kVA everything is relatively small to begin with so that may not end up being a big hairy deal either. Lastly because of the lower conductivity of Al and that resistance changes more with temperature, Al transformers tend to be slightly less efficient. But again, at 45kVA that is not going to end up costing much over it's lifetime, especially if both versions meet the DOE-2016 (DOE 10 CFR Part 431) efficiency requirements . Bottom line it's probably a preference call.
