Ahh yes the infamous Groupe Sportif. Even if you managed to persuade them to bring in the 3.1 rim, you'll need to use the specific eyelets, which need to be loctited (trust me), aswell as specific nipples! Too much hastle.
On another note, If you flat spot alot, go the 521's.
If not, the 321 is slightly stiffer and very stable on cornering.