Choosing the right bike size I'm looking to get the Trek Domane 4.3 and debating between a size 54 and a 56. One shop I've been to told me I'm a 56 and another said 54. According to Trek's sizing chart I'm 56. 
I am 5 9" with an inseam of 30.5". Which is the right size for me? Btw, I prefer not to replace any parts or make modifications to the out of the box setup. It should also be noted that I prefer comfort over performance.
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.