Both, or 'it depends'. BS 7671 is 'the regs', while the OSG is a distillation of those regs, along with some practical advice. If you are training to become an electrician you should have both anyway, along with a whole lot more. If you are a keen DIY-er with some technical know-how then the OSG will probably be all you need.
As far as my own experience as a (reasonably well informed) DIYer is concerned, w.boy, The Electricians Guide was my first purchase.
I found that this could be read like any other technical text, i.e. I could settle down to read a chapter, and by the end of it be fairly sure if I had, or had not, understood what was going on.
When I felt the subject matter and scope of it was a little better under control as a learning exercise, I got the OSG. This is far more like a list of instructions - with tonnes of tabulated data and schematics. I feel that it would be quite a challenge (at least for me) to read this book without something like the Guide to put all the information into some kind of perspective.
Just my own view mind - I am sure there may be those with different teaching and learning methods.
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