Most logs will split with a 5" cyl at less than 1000 psi. On my splitter most logs don't even kick the pump into low speed. Occasionally there will be the big old crotch that you have to be strategic to split at 3000 psi. I have stopped a 30 ton splitter on a crotch but you have to try to do it. If you read the wood and are strategic on your splits you can split most anything with a 20 ton splitter. Before I built mine I used to borrow one that had a 4" cylinder 2000 psi and a 30° straight wedge that was 6" wide at the back. There weren't to many pieces that we didn't split.
That being said GPM's are your friend. They are the most expensive to implement as more gpms means more hp, larger hoses, larger control valves, larger tank etc.
Farmall Guy, I just added an autocyle valve to my splitter this summer along with a log lift and hydraulic 4 way. I will have to be a LOT more carefull than the old setup which had no detents on the valve.
Don