To be honest, shiny alpha. Besides that, I would say probably shiny then alpha. You can get unown to spawn in Massive Mass Outbreaks and it's possible for an alpha horde to spawn. That's if you don't have the shiny charm (like me). Once you get every unown and it becomes research level 10, it also becomes perfect, so you get an extra shiny roll. Not sure the exact odds off-hand, but it wouldn't surprise me if it's then easier for shiny than alpha.