Good question! As a licensed Mortgage broker, I always tell people to try and figure out how long they intend to stay in the home. If you are looking to move in another 3-4 years, a buy down is not worth it. If this is a home you see yourself staying in for many many years, I recommend the buydown.
As for the buydown, 1 point= 1% of the loan amount. If you are doing 2 mortgages, keep in mind that the buydown is 1% of your first mortgage amount, not the total purchase price.
Usually, if you pay 1%, this will generally knock you interest rate down from.375%-.5%. On a small mortgage of 100-200k, this generally wont make a whole lot of difference in your monthyl payment. If your loan is larger, than the buydown would be more beneficial.
Some simple figures to consider... Lets say you are financing 250k. With good credit, you would qualify for a rate of 5.875%. With a buydown of 1% ($2500), this would lower your rate to 5.5%. So at 5.875% on 250k, your monthly proncipal and interest would be around $1479. At 5.5%, the monthly principal and interest would be around $1419. So you would save $60 per month. Now what you do is you take the amount of the buydown (2500) and divide it by the monthly savings (60). This will tell you that you will have to be in the house for 42 months to get your money's worth out of the buydown. Let me know if this helps out! Good Luck!