You can do a search in these forums and read alot about the dfiferences between brands. However, your B7800 vs JD 4210 is still a good question.
As you know, the JD has a larger frame size. You havent filled out your profile, so I cant tell how much property you are trying to manage....
I went with a kubota and the kubota dealer is 70 miles away. JD dealer is 6 miles away.
I went with the tractor I liked (kubota), NOT the dealer I liked.
I figured Id spend a lot more time on the tractor than I would at the dealer! Seriously, BOTH are excellent machines. Go with your instincts!
One comment I might make.
My friends around here that have compact tractors usually are divided ths way.
Growing up in agriculture as a kid - they go with a John Deere compact.
Growing up in the suburbs or hobby farms - they go Kubota compact.
Im the exception. I was a farm kid and bought a kubota. Either would be a good choice.
good luck.
dwight