Is France for me?

Your major problem, whether in France or Spain, is that you don't have an automatic right to residence and employment, and you need to get the required visa in advance. Opportunities for this are very limited for US citizens (just as they are for Europeans in the US), as you need a work permit and one is only issued if your potential employer can prove lack of suitable applicants from within their country or throughout EU. Perhaps the easiest is if you work for a corporation that has offices or subsidiaries in either country and apply for a transfer, when all paperwork will be handled by your firm and there is less rigorous requirements to satisfy (e.g. if you are replacing another expat staff member), but you need to have been working for them for some time (usually 2 years), in a more senior position, with expertise and responsibilities.
Because of your youth and inexperience, you probably won't qualify under the normal work permit scheme, so you need to explore other avenues. You can, for example, do further studies and work in spare time (but you need to pay tuition fees in advance - look out for scholarships) or look for some kind of internship/work experience opportunities (contact your professional organization for possibilities). WWOOFing may be an alternative. Lastly, you can marry a national from an EU country or become a live-in partner, but procedure has been tightened as many people use it as a ruse to get round immigration restrictions.
The only European country with which the US has an agreement on working holiday visa for under 30s is the Republic of Ireland. Otherwise your stay in France, Spain or other Schengen countries is limited to 90 days within 180 days in total, with no work permitted.

As Joppa already said, your main issue with France (or any European country, for that matter) is going to be getting a long stay visa. For that you'll basically need a job, and jobs of any sort are kind of hard to come by right now.

As far as your other interests and requirements - political orientation in France is considerably different than in the US. What you consider far-left/radical may or may not put you on the left in French politics. And in any event, you have to have lived here for five years or more before you can take French nationality and start getting involved in the politics here. There is considerably more regulation (of everything) in France than in the US - something most Americans find a major irritant, at least at first.

And the state of what you call "organic agriculture" (called "bio" here) may not be quite what you expect. You might want to try and plan a holiday trip over here toward the end of January, beginning of February when they hold the big Forum d'Agriculture in Paris. That would give you some introduction to the agricultural sector here in France. They are powerful and militant, and while the bio portion is gaining ground, they're still a pretty minor player at the moment. I also kind of wonder about your ability to live "car-free" if you're looking for work in the agricultural sector in France.
Cheers,
Bev