I have the concentration of a moth at work today, so why not focus on something that actually interests me- steel tubing and bike geometry!
The 48cm Smoothie has 12mm of less reach than your current bike, but it also has 17mm less stack. Basically, to get the bars on the Smoothe to be the same as your Roubaix, you would need more spacers. Not the most elegant look, but if its within the established safe limit(about 40mm of spacers or less), then so be it. There would be a good bit more saddle showing too on the 48cm frame.
I have no idea what your current setup looks like, so I certainly couldnt say one fits better than the other, but based on what you have said so far, it would see the 51cm ES would fit best since almost all the measurements are effectively the same as your current bike and you could get the bars 3/4" closer with just a stem and bar swap.
To compare to a similar frame(double butted heat treaded tubing that is 8/5/8 in butting), the Black Mountain Cycles road frame in size 54 with the Whiskey + fork is 564mm of stack and 372mm of reach. Compared to your Roubiax, the wheelbase is within 1cm of length, the trail is a very close 62mm(32mm tire) compared to 59mm for your bike(25mm tire), the chainstay is 1cm longer than your Roubaix, and you will sit 'in' the bike a very little bit more than 'on' the bike since the bottom bracket drop is 75mm.
The Black Mountain frame will have your bars up 18mm higher(you could just use 18mm less of spacers to even that out if you want) and the bars will be 7mm closer to you. when compared to your Roubaix So if you used the Whiskey short reach bars with that, it would be 15mm less reach compared to your Roubaix without even changing your stem. Steering would basically be the same as you currently have, since 3mm of trail difference is very minimal. And the ride feel would would be a little more smooth due to the 1cm longer chainstays.
So many options and subtle differences to consider, right?