Solo1 is higher speeds and more chances of really crashing and hurting yourself. They are usually done on tracks or hills (hence hillclimbs). They usually have them at the Pagoda in Reading or at Hershey (for the Vintage Hershey Hill Climbs).

You also need more safety equipment and in most cases licensing too. You need a roll bar, fire extiguisher, nomex suit, nomex gloves, leather shoes, baclava (if you have face hair), SA helmet, and other things. It's still one person on the course at a time or when the person gets close to the finish they send the next one out like autocross.

Solo2 is autocross

I saw one car last year go right off the cliff at Pagoda in an Eclipse or Talon. I went