Originally Posted by
Quiglesnbits
A steeper seat tube angle (closer to 90 degrees, or more vertical), means that the seat will be closer to the bottom bracket, which would make effective top tube shorter without affecting reach as measured from the bottom bracket.
Likewise, a shallower angle (tilted back towards the rear of the bike more, or less vertical), means the effective top tube length longer, but still not affect the reach as measured from the bottom bracket.
I am by no means an expert, but my interpretation of what it really means for fit is how your center of gravity will be placed relative to the BB. As the seat goes back, your center of gravity goes back for two bikes of the same reach from BB, but you also increase the angle of your chest to your arms, which makes you overall lower.
+1
For the sake of comfort you want to sit back behind the BB as this takes the weight off your hands/shoulders. The Salsa 54cm has higher stack which is generally considered more comfortable vs a lower stack which is generally considered better for aerodynamics although the truth is more complicated then that.
The 52cm Salsa is pretty much the 54cm with the seat pushed forwards which isn't comfortable for longer distances. If you like the 54cm Salsa stick with it. The differences are primarily in the geometry.