For the past year, I have been loving my 8'10"Ghost which I bought at age 74 to replace my beloved 8'8" SP25, which a few years ago replaced my beloved 8'7" Flow V1. As I get older with more injuries and replacement parts, I find that I like to add a little volume every few years to keep the wave count and enjoyment level up. On the really wonky days on the 8'10" Ghost with big chop and cross-rollers, I sometimes have to give up when friends my age on bigger boards are able to hang in there. That motivated me to buy a 9'1" Ghost just to be able to stay out longer on those wonky days. After a couple of weeks of comparisons, here are the differences I have found between those two sizes of the Ghost, none of which are very surprising.
1. The 8'10 is a faster paddler due to being narrower. The width of the 9'1" pushes more water which slows it down and makes paddling more work. When riding a non-breaking roller into the beach, the 9'1" will drop off the plane sooner when it starts to slow down and push more water. 2. When in wonky water conditions, the 8'10" requires a lot of leg twitches and paddle action when standing still, when the 9'1" feels stable as a dock which means it accomplishes my goal of getting it to stay out longer on wonky days. 3. Both are excellent wave catchers, even on the tiniest one foot waves. More similar than I expected. 4. Once planing, the 9'1" is faster and more fun on knee high waves due to the wider tail providing more sustained speed while staying back on the tail for maneuvers. The 8'10" is faster and a lot more reactive on waist high or bigger waves which makes it a lot more fun in that size and up. 5. In chest high or bigger, the 8'10" handles later drops a lot better. The 9'1" prefers to catch waves earlier which it is good at. 6. Both boards will crank impressive bottom turns, but the 8'10" feels quicker and snappier. Cutbacks on the 9'1" feel sluggish compared to the quick response of the 8'10". The 8'10" generates a lot more speed from pumping down the line. 7. The biggest surprise was going out through whitewater or pitching waves. For this, I prefer boards with narrower noses that will punch through the wave rather than wanting to climb sharply up and over. The SP25 was great at this because of the very pointy nose. The 8'10" does a decent job of punching through, but the 9'1" lurches up and over everything which throws me off the board going out over whitewater or pitching waves that would be no problem on my other boards (the 9'1" is the biggest boards I have ever owned.)
The bottom line is that my new 9'1" is now my preferred board for knee-high surf or wonky conditions. The 8'10" wins in all other situations due to much quicker and more reactive performance on the wave, and faster/easier paddling speed. As I said, no big surprises (other than the problem with paddling the 9'1" out through whitewater or pitching waves which woud probably be less of an issue if I had more weight to hold the board from flying up in the air.)