Help with choosing a bike and a size
 

I'm looking at 2 bikes the first is the bianchi IMPULSO 105 COMPACT and the second one is Scott speedster 20. What bike is better? And what size should I go with 55 or 54 I am 5'9

Go to the shop(s) that have them, ask them to make minor adjustments for you and test ride both. Buy the one you like better.

Honestly, there's no way for us to say because sizes interpret differently and there's no substitute for how the bike feels to you.

If you are between sizes, it is almost inevitable that a smaller size will fit better. Even though that will result in a racier fit, the reality is that a short stem on a bigger bike will still have more standover and twitchier handling; you can always raise the handlebar if the bike has too much drop, but lowering the top tube or making it shorter is a whole lot tougher.