The m5640sud is listed at 3858lbs, the mx5100 hst is 3774, and 3620 with gears. They also have the exact same displacement. I'm not saying they are the same, but that's actually less than a few hundred pounds. I didn't know about the 5640 when I bought my MX, but I sort of wish I had. Build your Kubota only listed the 5640 as $1000 more than my 4700. Now, there's no hydro option, so I'm still happy with my decision, but it would have been nice to try both.
i'm with you on that & am sure the OP appreciates your input hope you continue to be content with your tractor...the mx5000 was a no nonsense workhorse for me that anyone could be proud of