Thanks for reaching out!
I have been paddling a 12' rigid board for almost 9 years now. Initially, I got a fixed length paddle, but as my family grew and we paddled more together we also added an adjustable to the mix to accomodate different sized paddlers. I have used both paddles, however, I prefer the fixed length paddle as it is lighter and feels better in my hands. I am 6' tall and use an 80" paddle. Both paddles have a blade size of 91 square inches so I can't speak to the difference there, however, I have never wanted for more power when I'm paddling. I'm a big guy with a big SUP and the 91 square inch blade works well for my needs. If I was paddling open water or fast rivers I might consider a paddle with a bigger blade, but I stick mostly to lakes and slow rivers so mine works great.
Hopefully this helps, thanks!