Originally Posted by Campag4life

I missed the geo metrics in your opening post.
Here's my take. The 48 and 51 bikes are almost a wash in top tube...only 5mm different...easily tunable with stem length. Since the bikes have the same 74 deg seat tube angle your reach and weight distribution fore/aft for all intents will be about the same on both bikes. There is a 20mm difference in head tube length. This is a game changer and why I recommend the 51 so you won't be too bent over. Bikes change more size to size in terms of frame height than they do reach...even though height affects reach fractionally. With the 51, worse case as your riding develops...you won't need to go any lower than slamming your stem and running negative rise and you will appreciate the longer head tube if you prefer a more upright riding position.

Before you pull the trigger on the Moto though, ride some bikes to reinforce your decision.
Below is the chart for the Roubaix considered a comfort or endurance geometry.
Note that the 52 Roubaix has tallish 145mm head tube versus 130 and almost the same top tube length of the 51 Moto.
Give the 52 Roubaix a ride...have them flip the stem down.