As everyone has said, the only difference is hp. The HP difference really only comes into play at the PTO, for the most part, because the CK25 can't be stopped when pulling or ground engaging. You'll almost always lose traction before you run out of hp. However, mowing or using other high-torque/rpm PTO attachments, like a large tiller, is where the CK30 has the advantage. So, if you are not using the PTO for such tasks to any large extent, the extra $ for the CK30 buys you very little extra capability. I have no plans to mow with my CK and am mainly digging, hauling, or pulling. That is why I opted for the CK25. I spent the money saved on implements. Now, if I ever sell it, the CK25 will have a slightly smaller market to draw from because of its lesser PTO capabilities. But, I have no plans to sell. My kids will have to bear that burden when I die.
In general, today's machines have way more hp than they need. Far too much emphasis is given to it. This is driven by competition between manufacturer's that use it in marketing hype to sell the consumer. In many cases, it's far easier (read cheaper) to add a fuel injector or the like to boost hp than it is to put a heavier duty frame, axle, or tranny in a machine. Further, the tranny/frame upgrade is harder to quantify in terms that the consumer can relate to. For many tasks weight, gearing, frame strength, and hydraulic capabilities are more important.
So, if you really need the hp, then get it. If not, buy other things that you need with the money instead, be it an implement or a pretty neglace for your better half.
Tom