Sizing Help - Niner RTL 9

Trying to figure out what size bike to order.  To date I've solely ridden mtb bikes, thus I'm not sure what size drop bar bike I need; unfortunately there are no Niner dealers in my area.  I'm 5'8", Niner website shows 53cm fits 5'5" - 5'9" and 56cm is 5"8'- 6'; I'm right in the middle.  

Gut tells me 53cm, but I have zero information to base this on.  Any words of advice?

Thanks & Cheers!

Go with gut. If you're in-between always go size down. It's easier to make a small bike bigger, than a large bike smaller through component selection

I'm 5'9' with 31 in-seem I  have a 53 and it fits very well...

Niner's sizing advice seems a little off.  Their 56 is a "big" 56, because the top tube is actually 56.5.  That is awfully long for someone who is 5' 8".

Many thanks all, I think we have solidified the decision.

Cheers!

I'm 5'8", ride a 52x53 road bike, and bought a 50 cm Niner RLT 9 Steel.  And it fits great.