can i also shoot 9mm?
Yes, 9mm aftermarket barrels are available for the P226.
should i get the sig in .357 or 40?
Personal preference. Both calibers are effective. There are several factors to consider. The .357SIG round is faster and has more energy than the .40, but makes a smaller hole (the .40 bullet has a larger diameter).
are the internals the same
Internals are all the same. You should be able to use the same magazines. Don't need to change springs or anything but the barrel to switch from .40 to .357SIG to 9mm.
Will i get more sight radious with one over the other?
Since the slide is the same, the sight radius is the same.
See above. An additional consideration might be price and availability. .357SIG ammo is more expensive and somewhat more difficult to find, likely due to the higher popularity of the .40 caliber over the .357SIG.
Hope this is helpful.