I haven't had the problem of the impact wandering as the barrel heats up with any recent Weatherby rifle, with the minor exception of the Ultralightweight Mark V. My ULW .240, however, shoots 3 shots in tiny groups before starting to wander, which is plenty in hunting rifle.
I would also disagree on only buying Weatheby calibers in Mark V's. I have a couple of Vanguards, one a Sporter (walnut) in .257 Wby. and a Sub-MOA in .300 Wby. that both shoots great. And just about all the Vanguards I've shot have also been very accurate, no matter what the chambering.
That said, the Sakos are great rifles and very accurate. They tend to have better triggers out of the box than Weatherbys, but that has changed somewhat with the most recent Vanguard triggers.
Both are very fine rifles and I would base the decision on other features like weight rather than the brand.