Choosing the right bike size
There are a ton of info online!

for example:
 http://www.ebicycles.com/bicycle-tools/frame-sizer 

Performance usually mean right size and also same comes for comfort. I don't know why you think that this things are in opposite ways. 

If you feel like 56 is too big and 54 too small go for the smaller. Also what use you will give to it ?  Smaller frames are better for more aggressive driving like in a urban enviroment. 

I know some cases when in a store seller advised wrongly in order to remove some last year(o more) stok. So be careful with this. It may happen in some small stores where the seller is also the owner.