Over in JD Buying/Pricing I have a thread "My 4310 is on the Ground" is where I debated that. Most of the advice was that you can live with the 420. But I had wanted the 430, and while it was new, was the best time to settle it.
You can read all the spec differances there, or look at them at Deere's site. There is only a little more strength, and reach, but that is what it takes to make it work, for me.
Here are the differances in use, from Deere docs:
4n1 Bucket - 430 Yes, 420 No
HD Bucket - 430 Yes, 420 No
Bale Spear - 430 Yes, 420 No
Front Blade - 430 Yes, 420 No
Large Bucket - 430 Yes, 420 No
Green Color - 430 Yes, 420 Yes
Some of that might work on the 420, but not by Deere's docs. Doing so, could void a warranty, even if it did.
For mev it was worth the little differance in price.