As a 425 owner who lives on steep, wooded, rocky clay that doesn't grow a nice lawn, here are my thoughts:
AFAIK, the differences boil down to:
1) 3 more hp.
2) 30-50% more claimed wheel torque, depending on who you ask and which models you compare.
3) 8" more lift height.
4) ROPS.
5) 2 steering cylinders.
#1&2 aren't very important unless you're going to be digging hard, working on steeper slopes, carrying or pulling heavy loads, etc. Since those things are most of what we do, we went for the 425. If most of what you are going to do is mowing flat yards, this may not be important.
Several 422 (and older 425) owners have commented here that they wouldn't mind more oomph and lift height. I haven't seen any 425 owners regret their choice between the two. In fact, there has been a good bit of discussion about upgrading the power and lift of the 425.
For me, #3 is a huge difference. I can just barely load over the side of my old Toyota 4x4 pickup. I'm pretty sure I could also load into a fullsize 4x2. With the 422, no way.
#4 is only important if there is any likelihood that you will ever work on a slope steep enough to risk a rollover, or where there are overhead hazards like tree branches. With a 422, this is a $500 upgrade (or $125 + shipping for the used one PT has listed at the moment).
My feeling is that #5 isn't really a big deal.
Our decision was that it made more sense for us to spend the extra upfront for the 425, and defer some implement purchase until later. I'm pretty sure it would cost more than the price difference to upgrade the 422 to 425 specs, and you'd have negated any initial savings.
As always, your mileage may vary. What works for me may not work for you.
Whichever you choose, I'm betting that you will find it's more fun than a basketful of puppies. Enjoy!
Gravy /forums/images/graemlins/laugh.gif
Edit for another comment: If your tractor budget is absolutely fixed, with no expectation of future additions, you may get more capabilities for your buck with a 422 and a few more implements. Just remember that a machine that won't do what you need it to is a waste of time and money...