wow those sets are pretty cheap. Rainbow systems are MUCH more money.
To be honest, Rainbow is generally used for commercial applications where you have maybe 20-30 or more kids playing on them everyday.
I think the smaller ones would probably suit you just as well, but it doesn't matter how big it is. The real issue is safety. Depending on the age of your child, make sure their is proper railing on the second story, that it isn't too high in case your child falls, and that the wood is sanded well and treated for life. you don't want it cracking or splinters. In addition, make sure if there are 'bridges' or on the railing, that their appendages will not get stuck in various grooves. I think that that is one benefit of a rainbow system is that they have been in the business a long time, they are just some carpentry team making the products, but i think any company will be fine as long as it is made well and more importantly, designed with safety in mind.