can anyone tell me which would be better for my westie pup, a harness or collar ??

It's always best to try and start off with a plain collar.  IF you then have problems getting your dog to walk to heel, then you could consider using a 'stop pull' harness (not one of the ones with thin cords going under the dogs armpits please - these cause the dog pain :( ) simply to excercise the dog, but train using a collar.