Congrats on your job offer. I started last year with a big 4 also in audit and have thoroughly enjoyed it so far.
You are obviously well within your rights to turn the offer down however I would seriously advise against applying for additional jobs for one key reason. A friend at uni was actually in exactly the same situation as you and decided to apply for further jobs and and was offered a position with an IB. However a major problem arose in that both companies he had offers for used the same pre-employment vetting company and obviously they found it strange that they have 2 screening jobs for the same candidate for two different companies who was due to start at both. To cut a long story short they contacted the employers to say what had happened and he subsequently lost both offers and is now jobless.
Obviously that’s pretty worst case scenario but there really is no way of telling what screening service each company uses (the major grad employers only really choose between 2-3 major vetting providers) and if they happen to be the same then you will have some serious explaining to do.
If I was you I would stick with the audit job and enjoy your time off, you’ve done incredibly well to gain employment considering the current economic conditions. Yes it’s not the most glamorous work but if you gain exposure to financial services and capital markets as I have then you are primed to enter IB’s etc when you qualify with ACA.
On that note if you do decide to stick with your 2013 offer then I would really advise you spend some of your year looking through some pre ACA stuff and brushing up on basic maths skills, it’ll really help and give a massive head start.