Deere is not heavier built than Kubota, it is built differently leading to a machine that simply weighs more.
Kubotas are known for being fairly light machines, especially the non-ag tractors. Ladder frame construction is part of the reason why they are lighter. The claim is they are just as strong as a traditional design where the engine/transmission make up the structure.
Don't know the pros and the cons of it, but at least for many users of CUTs, a heavier tractor is not always a desireable feature when you have to drive it across a lawn. For ag purposes, weight can become a serious advantage.
As for Deere being in a different league, I think that is open to debate. Each of the Big 3 has its differences in design, but I don't see an overwhelming difference between any one of them. The best front end loader machine is probably the New Holland right now. The best CUT lawn tractor might be Kubota. For ag tractors, I think that Deere is probably the best choice. But each brand has its quirks, and each model may be outclassed in one or two ways by another brand's machine; to say one is the definitive best brand is simply personal bias. There are some other brands that are coming on very strong with high quality offerings, Kioti really stands out as the NEXT big brand to be.
I think you would be best served to look at the specific tasks you want to accomplish, then figure out how much time will be devoted to each task as a percentage of the total time, then pick the best compramise machine to do those tasks in the most objective and color blind manner possible.