I'm a developer that has always done custom XHTML/PHP sites. I'm starting to get more requests from people wanting simple sites for them to be based on WordPress so they can easily edit and add content pages for their business site.
I'm also planning on doing some product brand enthusiast sites for SEO purposes.
I've read a lot of discussions about Genesis and Thesis but there are usually two problems in the responses. Either 1) people commenting only have extensive experience with one or the other so they really can't contrast the two, or 2) most people using these themes are looking to avoid any coding or having to learn CSS, etc.
Some may say, well since you are a developer, you don't even need these frameworks, but I think it would be useful to base our WordPress sites on one of them should a client decide to 'go it allow' with managing their site including the layout and design. For my clients though they will be original designs. Not some theme that anyone else can purchase and then just change a few colors/fonts and the basic layout. For the enthusiast sites they can be various cookie cutter themes. Don't have to be original designs.
I've read one advantage of Genesis (which many don't point out because they aren't developers) is that unlike Thesis, if you upgrade WordPress and/or the framework you don't have to re-install your customizations because you can just modify a child theme. Where Thesis you have to backup files and reinstall them.
Anyone with more of a technical perspective that has used both can comment on how the compare. Of the people commenting that have experience with only one or the other, you're always going to see more recommending Thesis simply because it has been around longer. Older isn't necessarily better.
However I'm concerned that I see some consistency in comments saying Thesis has better on-page SEO capabilities (and results) although can't you use a plug-in with thesis (like Yoast SEO) to overcome any SEO limitations for Genesis or does the latest Genesis version have built-in SEO functions that are now as good or better than Thesis?