I was in your opposite situation, French student, gone to Berkeley, in good part for the cultural enrichment and international opportunities.
So I see exactly where you're coming from. I think your plan and perspective is *very* sound. Chapeau. Vas-y mon vieux, tu ne le regreteras pas. You can still change your mind after one year at the Sorbonne, it would have served as a fantastic cultural experience at the academic and personal levels.
Yes the Sorbonne is less prestigious *in France* than l'ENA, Normale or Polytechnique, and all other Grandes Ecoles, but it's actually the biggest French educational brand outside of France. American employers will be impressed. Besides, all the Grandes Ecoles are way too "franco-franšaises" (that's one of the main reasons I went to Berkeley), except maybe the HEC.
As well, ignore the advice relayed here but nottooold, about a French person recommending UCLA over the Sorbonne. What they fail to see is the incredible cultural enrichment that such an experience would be for an American. And in France of course, the grass always looks greener abroad...
I would have been less enthusiastic if your American options were better, but UCSD doesn't strike me as a great place, it comes across as a commuter school, definitely not the best in an American undergrauate experience, particularly if you're not interesting in Bio research, and particularly since you're probably from southern California.
Sorbonne, Ó fond.