Heya Vanessa

I have to say I have very little experience with roses but if you go with the cover crops
I would guess that there will be some competition between plants, however your overall biomass should be greater than with just the roses and mulch, which I believe should result in a greater carbon sequestration and benefits over the long term.

Perhaps check out Christine Jones keynote speech where she talks about the Jena Experiment
https://youtu.be/V4uVKIGBk2s?t=3040