I'm from MA and I want to buy iPhone 5 unlocked, is it cheaper if I buy it in New Hampshire?

Unlocked means unlocked.

If the sales tax rate in whatever city in MA is less than the sales tax rate in whatever ciry in New Hampshire, it will cost less to purchase the iPhone in the city in MA, but you aren't talking much here.

The warranty for the iPhone purchased in the U.S. if needed will be honored in the U.S. only. You should wait and purchase an iPhone in Spain.

Sales tax will be the only price difference. Phesant Lane Mall, off of Rt3, Nashua, NH and Rockingham Park, off of Rt93, Salem, NH are the 2 New Hampshire Apple Stores I'm aware of.

As Allan Sampson says, the Apple warranty is only valid in the country where purchased, so taking it back to Spain will mean either returning to the US for any warranty work, or financing the repair yourself. I guess this would depend on the duration of the iPhone warranty and whether you decided to get Apple Care extended warranty or not.

The US available GSM iPhone 5 will not work on LTE in Spain only 3G

http://www.apple.com/iphone/LTE/

There may be a few dollars of difference, certainly not enough to cover the cost of gas.

The warranty will not be good in Spain.

For clarity US Apple Care + is only available and valid in US the rest of the world only has AppleCare which again is not transportable crossborder

Keep in mind as well that it will NOT work on LTE networks outside of North America.

The warranty is for one year but you cannot use it outside of the U.S.

As several people have told you in this thread, the warranty is not international. The warranty is valid only in the country of original purchases.

The warranty doesn't automatically expire if used overseas. The warranty will be honored in the U.S. only.

Plenty of iPhones are exchanged under warranty. Hardware can fail at any time.