But, it only makes it unbreakable for 50% of passwords? Would it not be better to have an alternating scheme, i.e. MD5 -> SHA1 -> MD5 -> SHA1 repeated 100,000 t

But, it only makes it unbreakable for 50% of passwords? Would it not be better to have an alternating scheme, i.e. MD5 -> SHA1 -> MD5 -> SHA1 repeated 100,000 times, rather than choosing a scheme deterministically at the start.