If you hire everything out to repair the 100 year old house then it could work out cheaper to just demolish the old house have a new one built.
But if you do a lot of or most of the work yourself then it's cheaper to repair the rot in the old house.
Plus you don't have to have a new build permit which equals more money and older houses tend to have lower real estate taxes but if you build a new house you might be paying $4,000.00 to $15,000.00 per year in real estate taxes depending on the State your house is in.
I'd take the older house with lower property taxes and repair it to like new condition inside.