I was going to say the M4800SU. Apparently they're available in both 2WD and MFWD, although Kubota.com only specs out a 2WD. It's a little more robust than the MX5000 and has the planetary final drive like (I believe) the 5103 has. Still, the 5103 appears to be a much heavier tractor and it's 179ci engine will certainly produce more torque than Kubota's 148ci.
The M4800 transmission is 8x4 while the JD has a 9x3 in its most basic config. Both have 14.9x28 rear tires (R1) and the 3PH lift capabilities are very similar at around 3K lbs. The Mx5000 has 26" rears and significantly less 3PH muscle.
However, the M4800 probably runs about $3K to $4K more than the 5103. 'Made in Japan' can't compete with other Far East mfg on price. Looked hard at a MFWD M4800 this summer; never got the quote 'cuz it was too early to jump. Would like to replace my two oldest tractors with one.
Bob