What sort of texture do you want? What done-ness? How do you plan to finish it?
131F will give you medium rare. 36-48hours should give you a slicing, roast meat type texture. You'd get a bit more tenderness to cook it unrolled, just because it won't spend so much time coming up to temperature equilibrium.
Sous vide cooking is all about heat penetration and time at temperature. Once the meat is up to temperature the task of converting collagen to gelatin begins. This happens much more slowly at 131F than it would at 155F. So the same cooking time will give you a different texture at different temperature.
The reason @Brian1 asked about thickness of your roll was to estimate how long the meat will take to come up to bath temperature at its core (temperature equilibrium). This is one of the important parts of sous vide cooking. You can estimate penetration at around 1 hr per inch of thickness.