I grew up splitting wood with a maul and other than using it on the occasional log for smoking wood, I have no desire to use it again. I would split hickory and oak into quarters, ash into smaller pieces for kindling and any elm we cut was always used as "backsticks". Those you put on when you had a good hot fire going that would go all day or night. Elm does not split well at all with a maul from what I remember.
Now, I have a wood burning fireplace myself so we cut and split wood each year. I will attempt to show my knowledge/ignorance of log splitters. I've never used one of the store bought ones. My buddies and I ended up with one that was home built. This thing is built like a brick chithouse.....a 19 horse Kohler engine running a 2500 psi pump with two 4" D x 36" L Industrial Tie 2500 psi cylinders. I have seen this thing bust logs that were close to 4 feet in diameter (also have the electric winch mod to keep from straining your milk picking up the big stuff).
I do remember talking to the guy that built it one time and discussing the hydraulics and stuff. Yes, pressure is important but he said most of the store bought ones don't have a high volume pump on them. He told me what this pump was rated at as far as gpm but I can't remember. Anyways, you can have a 10,000 psi pump but without the right volume to support it, you'll be all day splitting one log cause it moves so slow. I know ours has a 10 gallon tank on it and it only takes about 5 seconds to bust through a log. So, volume seems to be just as important as pressure.