Here are specs on the 420 and 430 from a dealers website.
420 Loader
* For 4210-4410 Tractors
* Lift Height of 87"
* Lift Capacity of 757 Lb. To Full Height On 4200
* Lift Capacity of 859 Lb. To Full Height On 4300
* Breakout Capacity 2,141 Lb.
* Hood Guard Standard
* Quick Tatch
* Requires Dual Selective Control Valve on Tractor
430 Loader
* For 4310-4410 Tractors
* Lift Height of 101.4"
* Lift Capacity of 1045 Lb. To Full Height
* Breakout Capacity 3,299 Lb.
* Hood Guard Standard
* Quick Tatch
* Requires Dual Selective Control Valve on Tractor
The 430 lifts about 14" higher, 176lbs more to full height, breakout is about 1000lbs more and costs about $500 more. The cycle times may be faster on the 420 because of smaller cylinders, but don't know that for a fact.
I have a 430 on my 4410 and have already used its limits by loading a canoe on top of pickup truck and moving large rocks!