Replying to [Mankiw]:
T/T is stands for Telephonic Transfer, or Wire Transfer. While doing T/T, bank transfers the money without any specific inquires to find out who is the beneficiary, what is the purpose of money. T/T transfers money directly into the beneficiary's account.
T/T does not protect the buyer. And when paid in advance, buyer is on the mercy of seller. There is no mechanism which forces seller to comply with order/contract.
Where as under L/C, Letter of Credit, the interest of both parties are safe. As banks are getting involved into the transaction, the money are safe. Both parties, buyer & seller, have to comply with the conditions of L/C, & if everything is okay document will be cleared by bank & thus payment will be made to seller.
For more information refer to
http://www.creditguru.com/guestarticleLC.htm
Hope this helps you to choose.
Thanks
Krunal