Well I guess I'll be one of few CK25 owners to reply. The 25 does everything I need it to do on 7 acres I own. There hasn't been one time where I the job I was doing I felt like I needed more horsepower. Maybe more traction. Sure it would be nice to have a 90hp tractor and 10'+ bushog to work with. Heck if you have the money and that fits the bill, go buy that. But being as that you are looking at the CK25/30 you are most likely on a budget like the rest of us and those high HP type tractors are out of range. I agree with you the CK20 may be a bit small for a 20 acre job site, but it will still do everying you listed, and do a great job. The CK25/30 will do it slightly faster. With the 5 extra hp of the CK30 this will allow you to run a Landscape Rake, Finish Mower, Aerator and Tiller 1 additional size bigger then the CK25. So if that's a major plus on your checklist then the CK30 is more for you.
The remaining implements Kioti recommends to stay under warranty specs are all the same size. The FEL, 3pt, have the same specs for amout of weight they can lift, the hydraulic pump is the same 12.7gpm.
My FEL so far with 124 hrs does not have any cracks.
So far, I have put up a 3 board horse fence with a PHD. Digs through clay and rock just fine, just don't bury it to quickly or you'll be sorry. Bush hog 6.5 acres once a month in the summer. I use it to remove snow, move many tons of dirt, log trees, install culvert pipe, disc food plot, and help my neighbors out.
If you have the money go with the CK30, if you want to save a few bucks the CK25 won't miss a beat. Extra money could go towards implements.