It's got to be a decision based on which car you like best and which fits your budget.
Be warned you may find some Audi Dealerships come across as being a little rude, especially where the S3 is concerned as they don't really struggle to sell them.
Having just gone through the process of selling my S3 and ordering a new one I have spoken to many dealers. Some will offer no discount at all and others will eventually offer some. I have been lucky and found two dealers willing to offer well above what others have so I've made quite a saving.
I can also say that depending on what age of used car you are looking at you'll likely find it a much better buy to go for new if you have the funds. My 2 year old 3 door is currently at the dealership for £25k!!
