I can't really say, but that oak is a hard hardwood and alder is a soft hardwood. I suspect both work pretty equally, but I have observed that all the great beetles seem to occur in areas where there is oak. It may have more to do with the level of decay in the wood, than what type of wood it is. Still, I'd always choose oak if given the choice. The nice thing about alder is that it is sold in huge bags for snake/animal bedding. It decays in time and when exposed to moisture, etc. In captivity a lot of beetles are supplemented with bits of protein based foods, so it is difficult to know exactly what creates big beetles. Orin's books lay this all out, as do a few other books, but I suspect additional experiments would at least be interesting.