I've got a Nexus 7 and it's wonderful. The Kindle is nice but you're locked out of the Google App Store so to me it's not worth it. The iPad Mini is $329 vs $199 for the Nexus 7 16 GB models and for your $130 extra you don't really get anything better in the iPad, unless you consider iOS superior to Jelly Bean. For me that makes the iPad a definite no-go but it depends on your financial situation. Other than the price the biggest difference is that iPad is wider in portrait mode (more square) but the resolution is noticeably lower than the Nexus 7.
Another nice thing about the Nexus line of products is that they always get the latest Android updates right away vs. something like the Galaxy Tab. I've never used a Nexus 10 but I imagine it's like a huge, high-res Nexus 7, which is a good thing. I like the 7" form factor though, general web browsing is probably better on a 10" but the 7" is very portable and easy to hold in one hand. You might want to go to an electronics store to see how they differ physically. I have to recommend the $199 Nexus 7 16GB.