Both are valid approaches, and both often come up with the same or very similar answers. Even with the detailed approach, this is almost always hypothesis-driven i.e. in the first week of the project people guess what the answer will be and then set about analysis to determine whether they're right or not. To this end, both approaches are often influenced by the "gut feel" of executives and consultants, though you have more evidence through the detailed approach to support conclusions. Very often, executives can "feel" what's happening in the market, but they lack the detail of what customers, competitors, suppliers etc. are doing and so collecting evidence is valuable in updating their knowledge.
Proper corporate strategy should generally only be set every 2-3, the in between years more of business planning cycles than strategy. For me, the best way is to have one detailed review followed by 1 or 2 of the gut feel reviews to tweak operational strategy.
However, the strategy houses have made an art of convincing executives that everything they do must be evidence based (i.e. some spotty 20 something researches the market that the CEO has worked in for 25 years) to justify any decisions to shareholders. The fact that, as above, the detailed research will lead to exactly the same conclusion seems to be lost on many people.