I have a FN action and had it barreled in 240 Wby. The 240 Wby is one of the ballistic triplets. All three have the same case capacity and achieve the same velocities. The other two being the ones you mentioned. It has the correct bolt face. 257 Wby does not have the correct bolt face.
popular myth #1. Belted magnums are not accurate.
popular myth # 2 Wby brass is expensive
popular myth 3 FN are not accurate.
Here is a long discussion on 6mm and a picture of a target that shows what a 240 Wby is capable of on a mauser action in the hands semi idiot.
I would advise you to go to a faster twist than one in ten. it will help you stretch the gun out to its maximum potential with bullets heavier than 100 grs.
Die availabilty is about the same for all three of the big 6mms. In fact you will find the 240 Wby die listed in amongst the wildcats for many of the die makers. Redding just flat out refused to make me a custom seating die for it. I was P O'ed big time