I choose being immortal for the simple fact you could spend forever making sure no one EVER had a perfect year of happiness before they died. Sure that makes me sound like a cruel bastard, but it would really even out the playing field against all the suckers who chose to have the one year of happiness. What if the person who chose immortality was the one who was assigned to be the one to kill you when your year was up? Ohhhhhh... they could really make you wish you picked the immortality. How nice of you to have lived a perfect year, only to get teabagged and tortured for that one last day.
Also, if you chose immortality, eventually I think you could figure out how to become a Highlander. I know, I know... you have to be BORN a Highlander. I'm sure there is a loophole there somewhere, and if you had eternity to figure it out, I'm willing to bet that you could.