Odeon - well done; a very good time for your first "serious" marathon.
Have run in Berlin & Abingdon
Berlin:
Pros - flat, fast, former WR course, free beer at finish
Cons - in Germany, expensive trip, might be hot as it is nearest to summer
Abingdon:
Pros - flat, cheaper and easier to get to, you'll come in top 10, weather should be ideal
Cons - might be lonely, not high profile