First thing to get cleared up is that tonnage ratings are mostly fictional on consumer end splitters.
The best way to find true max. tonnage is to find the cylinder bore size. The cylinder bore size regardless of engine horse power will determine max. tonnage.
5" bore =29 tons.
4.5" bore = 23 tons.
4" bore = 18.5 tons.
Because of the max. rating of components even these tonnages are never attained.
Larger hp. engines will allow for higher gpm pumps that will produce quicker cycle times but will not change tonnage or gpm.
Normal combinations for consumer splitters are,
4" bore , 11 gpm pump, and 5.5 hp. engine.
4.5" bore, 13.5 gpm pump, and 6.6 hp. engine.
5" bore ,16 gpm pump, and 8 hp. engine.
Commercial splitters will often increase pump size for faster cycle times,
4" bore ,16gpm-8hp., or 22gpm-12hp.
4.5"bore,16gpm-8hp. or 22gpm-12hp.
5" bore ,22gpm-12hp. or 28 gpm-18hp.
For the average home owner they will ALL split almost everything . Probably the most important thing to look for is operator comfort. You don't want to be bending over all day splitting wood and you don't want to have fenders or tires in your way while working.