Don't listen to that Econ guy above, who basically forgot why Economics became so unnaturally competitive - while some Land Economists may be lazy and weaky, some Economists are just awful careerists interested only in IB, not in Economics - they apply having stellar grades and 'rehearsed' interest in the subject. I don't know what is worse.
OK, Economics is more competitive gradewise but please, Land Economy offer holders have now Further Maths, UMS scores over 95% and a decent number of GSCEs. This year at my preference college there were like 20 applicants for 2 places for LandEc - more than the avarage for Economics.
Believe me, there isn't any difference in interview difficulty. Last year I had the interview for Economics, this year for Land Economy. That one for Econ was very mathsy, but doable, with a typical article from 'The Economist'. When you apply for Land Economy, you can get a normal economics interview with maths involved, a human geography interview, or a law interview (it depends on the college).
My PS was not adapted to Land Economy. I tried to focus it on urban and development economics with a hint of macroeconomics. Cambridge is aware of that PS-adjustment problems and will take it into account, so you can have either economics or law PS - it is your choice.
With 3A* at GSCE you have to nail your interview no matter what course you're applying for. So it is your choice, you may get an offer even for Economics (ye ye, many of them don't even take Further Maths and get offers).
So it is your choice at the end, but don't apply to E&M - Oxford is a bad place to be at and they love their TSA and GSCEs so you may not even get an interview.