Aaron Ramsey had a similar choice between Arsenal and United, and he chose Arsenal. Professionals will more often than not do they what see as better for their career.
Sign for United and be the only left back at the club, first choice by default, at a time they're promising to spend big on world class players, and help shape the club for years to come under a new manager who will be given time, play alonside Rooney, RVP, Mata etc - up against signing for Chelsea to compete with Azpilicueta for a full back position, under a manager known for his short termism working for an owner also known for his short termism, play alongside Hazard, Oscar, Eto'o etc.
It's not an easy decision at all, there's negatives and huge positives for both clubs.