Hi
I personally would look at a 455 Deere with the 22 hp Yanmar Diesel, I have owned both a 425AWS and a 455AWS. I will say that both are a reliable mower, although I would check to see what year this 425 is your looking at is... Early 425's had a plastic cam shaft gear that tended to break or implode without warning. Some have had the plastic cam shaft gear go at 200 hrs and others much higher in hrs.
If it does have the plastic cam gear, I would take the engine apart and replace the cam gear with a metal cam gear that most owners did with their 425's.
Thus said, despite Diesel prices being a bit higher then gas, the Yanmar Diesel in the 455 is much more fuel efficant then the gas Kawaski engine in the 425. I have had both, and theres a differents in fuel economy. If your runing a 48" or 54" deck, the 425 is more suited for those sized decks. I had a 425 with a 60" deck before my 455AWS 60" deck and at times the 425 seemed a bit underpowered for the 60" deck... Granted it will run the 60" deck, but the power is not there if your cutting wet thick grass, with the Diesel this is not an issue, but if you always keep your grass cut short, the 60" deck would be fine,
I would still prefer a 455 Diesel over a 425 though, just my opnion but the 425 would still be a really dependable mower in any case and if the price is right.
Take Care Jason B