I went to PT to buy a 1430, and got an 1845 instead. On aoccasion, I wish I'd gotten an 1850, but the 1845 has done everything I've needed. There are some jobs that require bigger machines, and some where the bigger ones are faster. There are few machines more flexible and capable than a 425, however, and those jobs it can't do are far outnumbered by the ones for which its maneuverability and astonishing lifting power suit it.
The 1430 is bigger than the 425, the 1845 is bigger than the 1430, the 1445 is bigger than the 1845, and the 1460 is bigger yet. As you go bigger, you give away maneuverability and the ability, for instance, to go through narrow gates. You gain power and, as weight increases, pulling ability and lifting capacity. Unless you identify particular projects, I can't say what machine is "better."
I have a Deutz diesel on mine, which is superb. I have a bunch of gasoline machines, including a Kohler or two, and I do not believe that diesel is generally better, just because it's diesel. I have a gasoline engine in a boat that is now about 16 years old, with maybe 2500 hard but intermittent hours on it which has never missed a lick. Even if diesels last longer, which is always debatable, I won't. Diesels are more expensive, noisier, smellier, but the one in my 1845 is a gem. (I also like gas turbines, but don't have any at present. /forums/images/graemlins/grin.gif)
I'll be happy to demonstrate my 1845 to anyone who gets to Maryland. If you want comparisons, Tazewell is a good trip, particularly now, with the fall colors, and you can drive everything they have, dig holes, push dirt, and look through one of the better toy, er , equipment warehouses you'll find.