I just had to answer a similar question for myself and I went with a 7mm WSM. A 7mm/300 WSM holds the current British record for 5 shots at 1,000 yards and its bigger brother, the 300 WSM holds a 10 shot 1,000 yard record in the US so I wanted a WSM case. I went with the 7mm because of the bullet choices available in that caliber. This quote from page 167 of Bergers new reloading manual kind of sealed the deal.
"At the current time, the form factors for Berger's .30 caliber lline are not very good in comparison to other calibers."
And it looked to me like the 7mms were the best of the good ones.
In fairness, I already had a 6.5mm and 30 cal so I was kind of hemmed in. If limited to only the 2 you mentioned, I'd go with the larger cal (bullet choice), though I'm guessing whichever one you go with you will like it.