The best splitter for you depends on your needs by this I mean the amount of wood, how fast ,and how much are you willing to pay.
Cylinder bore size determines tonnage.
Pump gpm, gallons per minute, determine speed.
A good combination at reasonable cost would be a 4" cylinder with a 16 gpm pump. This will give you about 19 tons of force at about a 9 second cycle time assuming a 24" stroke.
Here are a couple other combinations for comparison. The tonnage is actual at 3000 psi rather than advertized tonnage.
4" bore -19 tons x 11gpm pump =13 sec.
4.5" bore-24 tons x 11 gpm pump =16.2 sec.
5" bore-29.4 tons x 11gpm pump =20 sec.
4" bore -19 tons x 16 gpm pump =9 sec.
4.5" bore -24 tons x 16 gpm pump =11.2 sec.
5" bore -29.4 tons x 16 gpm pump =14.2 sec.
There are some high end splitters that offer 22 and 28 gpm pumps that greatly increase speed.
The pumps used on splitters are 2-stage, 95% manufactured by Haldex/Barnes although sold under many names and the other 5% are made by MTE.
Engine hp. is determined by what is required to operate the pump at it's rated capacity.
11 gpm 2-stage pump at 3000 psi requires 5.5 hp.
16 gpm 2-stage pump at 3000 psi requires 8 hp.
22 gpm 2-stage pump at 3000 psi requires 12 hp.
28 gpm 2-stage pump at 3000 psi requires 15 hp.
Higher hp. than required does not increase tonnage or speed.