very interesting dilemma.
are you focused purely on finance? if so i'd consider the LBS offer. if not, i wouldn't bother...simply because 1. you have to wait a yr, 2. it costs more, 3. it's a 2 yr program vs. Oxford's 1 yr, and 4. your MBA's only going to get you in the door for an interview...what happens afterwards is up to you...and in terms of opening doors, LBS isn't going to do that much more for you than Oxford (especially if you want to work abroad).
the thing you hear about Oxford's reputation is that the further you go from England, the more reputable the Oxford brand name is. so unless you're dead fixed on doing finance in London, i'd say the advantages of a LBS degree < the disadvantages of graduating a full 2 yrs later than your alternative.
Oxford Round 1 - interview invite 10/30/09, admit 12/11/09, ATTENDING 2010
My Road to Oxford and INSEAD http://gmatclub.com/forum/my-road-to-oxford-sbs-and-insead-89413.html#p676253