The second one reminds me of Quadro which my neighbours had when I was little. It was fab as you could build stuff like this but then take it apart and create cars or houses. It is amazing stuff, think it is really expensive now unfortunately.
I am just a bit surprised that the dome farms aren't more widely available? Do you think its safe? the bigger one looks like it would last longer but 3m diameter is quite massive isn't it. it would fir in our garden but still...
We have an older version of the small one in plain green (the colours look way more fun) and I would say it is likely to be too small for the 7 yr old. My kids are 3 and 5 1/2 and the 5 yr old is starting to look a bit tall for it - mind you he is 96% for height for his age. They have really liked it though, so I would go for the big one if you can fit it in.
I think the reason they aren't more popular is a footprint issue - I've seen big ones in a few parks and they take up a lot of space on the ground. No safety issues here - we don't even have ours secured to the lawn any more and the kids have never come close to shifting it in use.