I say whatever trips your trigger - go for it. IMO I think the 7mm WSM will be around longer than the 7mm Rem Ultra Mag. I can't argue the 7mm Rem Mag is a great cartridge and deserves it place in the hunting world. But those WSMs are fine cartridges too in their own right. Give them time and they will prove themselves too.
Like the one post said, they've been around for about 10 years now and still going strong.