Welcome to the site, I can’t help much with the comparison as I do not know these machines. I did notice in looking up the Kubota that it does have a 4 year power train warranty so I imagine they are confident the tranny will last.
Not much difference between the K46 and the K58.The Kubota T2290 - tranny has similar torque to K46/k57- it has 3/4 inch axle. Honestly if you have hilly property and are looking for long term durability- I'd move up to next in line kubota with Hydro-Gear ® G730, or John Deere X380 ( K58) other option is for price of either of them new you can probably get late model ( lightly used) X500 series tractor that would last 30 years on your property
Hardly ever hear about a K58 going bad-- As for the difference of a K46 vs K58 ..Well if you dont consider-- 3/4 axle vs 1 inch axle, Powered metal gears vs Heat treated steel gears , max rear tire size of 20 inch on K46, vs 23 inch on K58 , 26 % more torque on K58 any difference,. I really cant help you!! To me that is a significant differenceNot much difference between the K46 and the K58.