If Legendary is as easy as Halo 3, Legendary.
If it is slightly harder, Legendary.
If it is like Halo 2, Legendary.
If it is like Halo: CE, Normal =P
I really hope they make it harder than Halo 3, the only reason I never finished Legendary solo on Halo 3 is because I hate the Flood.
Since we won't have that problem, Legendary all the way!