I like both cartridges a lot. The 240 is a not as common but I kinda like it that way. If you can afford to shoot it, do it. The WBY factory ammo I have used has been accurate in multiple rifles. However, another option would be to get a custom load developed from Superior or Copper Creek. I helped a friend get a load from Superior for his 240 WBY and it shoots awesome. After a sample pack or two, you then would have your load which you can continue to order as needed.
I would go with at least a 24" inch tube and 8 or 7.5 twist to shoot heavier, 6mm caliber deer bullets.
