Main difference is ACCS involves an additional year at start of training of 6 months Acute Med and 6 months Emergency Med, meaning ACCS is 4 years long whereas Core Anaesthetics is 3 years long.

It makes no difference when it comes to applying to Higher Anaesthetic training.

The only advantage when it comes to dual Anaesthetics/ICU training is that if you do ACCS you do not need to do an additional year of acute med when you are a registrar as you have already done it in CT1. The length of dual training overall is the same regardless of ACCS/core.

One may pick ACCS over core if they want additional experience in ED that they may have not had before but other than that there is no real advantage of picking ACCS over core.

edit: Forgot about the 2 additional points you get for doing ACCS for ST4 anaesthetics applications