As a non-designer that's not really an issue for me. There are so many cheap or FOSS design templates, snippets, icons, etc available these days that it's not a huge problem to put together a package of them for Foundation.
I use Bootstrap when
1. the default Bootstrap theme or a third party theme are sufficient
2. I don't need Foundation's better grid system
3. I'm using something other than Ruby (which for me is most of the time)
Foundation for:
1. There's a specific problem that their better grid system solves. I haven't come across this yet, but am aware of it for when/if I do.
2. Client wants a highly customized Ruby-based site with no hint of the Bootstrap look
As someone mentioned above, you can use Foundation and borrow design and/or javascript elements from Bootstrap and elsewhere if you need to go that route.
Could be just preference or even skills (my front end skills aren't that great), but I find many generic design templates to be not that great visually or a lot of work to adapt to a css framework like foundation. Time is expensive.
I like stuff with batteries included. Given bootstrap's momentum, it even has a plethora of design templates as well so you won't have a generic bootstrap looking site.
I use Bootstrap when
1. the default Bootstrap theme or a third party theme are sufficient
2. I don't need Foundation's better grid system
3. I'm using something other than Ruby (which for me is most of the time)
Foundation for:
1. There's a specific problem that their better grid system solves. I haven't come across this yet, but am aware of it for when/if I do.
2. Client wants a highly customized Ruby-based site with no hint of the Bootstrap look
As someone mentioned above, you can use Foundation and borrow design and/or javascript elements from Bootstrap and elsewhere if you need to go that route.