Our 420 is 20,000lbs with all the different options it has. The engine is rated right about 88hp, but however it is tuned, it is noticeably stronger than a 580SL. I don't know if the gearing is different or perhaps where the engine makes torque. I remember using 580s and when you met too much resistance, it would just stop and you really had to work to spin the tires. Even though rated for less power, our 420 requires an easier touch as it will dig with the tires in a hurry. To be fair, I haven't put it against the new Case M series, but then again I haven't used Cat's E series either so it probably doesn't make a huge difference in the end.
It is also feels more stable on slopes. Whether it actually traverses steeper stuff I don't know, but it feels better from the seat while working.
In Case's favor, they do seem to be idiot proof. There are tons of rental 580s that get beat on every day and just keep taking it. Now our 420 has almost 4,000 hours and still runs and dig great, so in the end, find a dealer you like and find a good price on whatever brand they carry.