St Andrews is a very good school. The fact that it's not in the Russell Group means absolutely nothing other than that they didn't negotiate with those universities who are in the group. They are better than more than half of the RG universities.
Saying St Andrews shouldn't be considered a top university simply because it's not in the RG is like saying MIT is not a top university by default because it's not in the Ivy League, when in reality their only actual rival is Harvard.
With that said, St Andrews's position on The Guardian's league table is biased towards their favour because of their being a Scottish university. They are very small, and so like the LSE, their admission scores got bumped up; that studying there is free for Scots and could lead you directly to a Master of Arts instead of a Bachelor of Arts also lead to more applicants than they 'deserve' to have. And admission score is a big factor in this particular table. Then of course the fact that His Royal Highness Prince William, The Duke of Cambridge having read that made them unusually popular among applicants. Keep in mind also, is that their table calculates satisfaction as well (in various aspects), and people who applied to St Andrews are likely to be choosing them because they do want to go there, instead of the equally good people who end up at schools like UCL, Imperial, or Durham who are more likely to be Oxbridge rejects and thus less happy about going to their universities to begin with.
Regardless of what the league tables say, you are still a lot more likely to get a better education at Oxford or Cambridge for the money they have got and the tutorial system they have. They also have the two lowest drop-out rates in the country.
Between the two of them, I don't really know which one is better. If you're going for prestige and name recognition world-wide, your only choices in the world are Oxford, Cambridge, Harvard, Stanford, and perhaps Berkeley and Yale; and between Oxbridge, Oxford is the more famous one globally. In terms of employment, none of these universities lead you straight to a CEO position unless you start your own company, in which case MIT might be your choice of studies. Cambridge has a slightly higher graduate salary but a lower employment rate compared to Oxford (ie more likely to have a higher salary but also more likely to not be employed at all); they seem to be slightly more sought-after in business, engineering, and IT; but Oxford has a better overall reputation among employers (according to QS). With actual CEOs and millionaires, Oxford tops Cambridge by a comfortable margin but I cannot see how that should be relevant to you.