I have a .250 AI in an XP-100. Nice lightweight deer and antelope cartridge - took an antelope at 320 yds with it. Longer neck than 243 might give you longer barrel life if you plan to shoot the rifle a lot. Factory ammo is harder to find for the .250, but to get optimal performance for your rifle and shooting needs you should reload anyway, so it is a non-issue.
At the risk of being an enabler, why not get both?
