Master Yi gets a lot of free health from his numerous heals so any amount of armor from getting a Randuins Omen or a Banshee makes you a bit more slippery. By all means go Block + Unyielding. I used to use that and I liked it. It is the superior option if you think your jungle is going to be invaded and it has more consistency.
I just like Enchanted Armor since I think Yi fucks with those squishy AD teams really hard. Zed or Yasuo | ADC | Support | AD jungle squishy or top | filler. Get a Randuins Omen and just win.
Neither option is great which is one reason why a lot of good Yi players IE Cowsep and a bunch of Koreans like to go 21-0-9. http://www.lolpro.com/season-4/1506-mastery-calculator#3vTha8vdtzUw
is more or less the standard benchmark to that style of Yi. Some people run Butcher or Feast. Some people don't run Phasewalker. Commando Yi runs Warlord with that but sometimes goes Enchanted Armor 21-9-0.
Slightly faster powerfarming and increased carrying capabilities with buff and summoner spell reduction. I think Juggernaut is too helpful on Yi, since it scales with Meditate and Dangerous Game and how balls deep he can be. 21-0-9 is very noticeably better if your team is doing well.
Edit:Cowsep seems to have a Warlord Yi page too. o_O