I don't know much about these basic items other than they are both "copper bearing alloys" (brass more identified with zinc and bronze more identified with tin?) and of course they have generically been around a very long time, even before any use as saddles! While not specifically involving saddles, I have heard of issues involving "dezincification" of some such alloys (containing zinc), as well as concerns of toxicity e.g. when any such alloy contains lead or other substance of concern that can be leached etc. into e.g. drinking water etc. I suspect these and other issues are probably addressed to some extent where necessary by reputable manufacturers e.g. by control of alloy compositions or limitations on exposure. I would think many such manufactures would also be happy to discuss their provisions for any such issues, and also I would think e.g. NSF 61 certification etc. (if the material is in contact with potable water etc.)
My only comment other than dezincification of the brass (already pointed out) is that bronze is probably a better choice as it is more ductile and less likely to suffer from fracture failure.
