Theme Forest & Elegant Themes... Difference?

Not sure about theme forest, but I believe it's just a marketplace where you can buy various rights to different themes from their respective owners.

Elegant Themes is a membership site, being a member allows use to all the themes they have available. Unless you purchase a developer license, they do not allow you to sell sites that use elegant themes (unless you remove the theme)

Thesis is a do-it-yourself theme framework. Experience with CSS is mandatory to make proper use of this theme.

ThemeForest is a marketplace. There many developers from around the world submit their themes. After the themes are checked by ThemeForest stuff they are approved (or not) for selling. Developers get up to 70% on every sale and ThemeForest gets the rest.

ElegantThemes is a company that develops WordPress themes. They sell them via a membership program. They cash in all the money and they developed the themes over time, that's why they afford to sell them at that price.

The difference in price is based on the business model, not on the quality of the themes. I used themes from both and I was very satisfied with their quality.

As for deciding where to buy from, it really depends on your needs. If you need more themes and find what you need in ElegantThemes then go with them.
If you need just one with more features (ex: an advanced theme for movie reviews, a theme for a music artist with playlists and events, a wordpress theme that doesn't look like wordpress) you will find it on ThemeForest.

Re Thesis I can't help, I personally don't like it, but many have good words about it.