Your main home is where you live most of the time. It doesn't have anything to do with which is newer or cost more.
It also doesn't really matter in this case unless there are more facts you haven't told us. You are allowed to deduct property taxes on all US property you own, whether it is your main home or a second home. And you can deduct mortgage interest on your main home and one second home. If you have three or more homes, you have to decide which two to deduct, but if you only owned two homes the interest on both is deductible and it probably doesn't make a difference which you designate as your main home.