A friends son has a left handed 700lss in 300ultramag and it easily shoots 1" groups.I own two 300ultramags myself and am very happy with them but they are suited more for specialized uses by a person with several rifles.By this I mean that they only offer an advantage over the 300win mag at distances of well over 300 yards.This advantage comes at the expense of increased recoil and reduced barrel life.To take advantage of the flatter trajectory at long range you must also be a much better than average shot and must practise a great deal at the longer yardage to maintain your shooting skills.Combine the recoil and shorter barrel life with a lot of practising and you can see that the 300ultramag is not a good choice unless you have other rifles of similar trajectories to practise with.The 300 win mag is a far more suitable cartridge than either the 300ultramag or 300win mag for the vast majority of shooters.
As far as your choice of scope,I would go with a scope in the 3x10x40 range as it can be mounted lower for better cheek fit especially considering the recoil.A maximum power of 10x is plenty for 500 yard shots on big game and a low power of about 3x is better for shot opportunities that suddenly appear at close range.