I'm using a belt sander to sand a 25 year old pressure treated deck to prepare it for sealing. Which sandpaper will do the best job most quick? I can't tell from the descriptions I've read, where everything sounds good, but would appreciate first hand experience input. Thank you for your suggestions.

There just isn't much difference between 35 and 40 grit.
Does a 25 year old PT deck really need any sanding at all?
And any raised wood will rip your sandpaper.
I think most people pressure wash it, let it dry, and apply the sealer.

I think I was unclear. My question was not about which grit to use, but what kind of sandpaper was best. There are various brands and qualities out there and I'd like to make this chore as easy as possible. I did pressure wash the deck, bleach, scrub, and rinse it (and let it dry), but a friend thought sanding was a good idea and started the project. The difference is amazing, but there's still 3/4 left to do and it's a lot of work, mostly done on hands and knees. There haven't been any problems with sandpaper ripping.

I believe that is way too coarse and it abrades the wood quite severely. I would use a 100 grit belt, Norton makes good ones.

3M 80-grit was used first, but wasn't up to the task. 3M 50-grit wasn't much better. We're using Diablo 36-grit now and the result is excellent, but it's still painstaking work. Just wondered if aluminum oxide, zirconia, or any other material is best. I don't know if there's a difference using "professional grade," either, and the prices are all over the place with both material and brand.

Truthfully, I've never run into anyone ever who sanded down a PT deck.
I just don't think it's going to stay smooth more than a year.
Even expensive cedar or redwood or mahogany or sappele or epee gets rough after one winter or a season of rain, never mind UV.
PT is so cheap, you just replace boards as they crack.
Brands of paper, you get what you pay for.

Truthfully, I've never run into anyone ever who sanded down a PT deck.
I just don't think it's going to stay smooth more than a year.
Even expensive cedar or redwood or mahogany or sappele or epee gets rough after one winter or a season of rain, never mind UV.
PT is so cheap, you just replace boards as they crack.
Brands of paper, you get what you pay for.

Yes, tend to agree with replacing boards as they wear and crack. Less work, safer, I just did that, and painted. Looks like new and no elbow grease

My friend took it upon himself to start sanding the deck as a way to give it a fresh look and it does look new where it was sanded. The rest is quite dark in comparison, even though it was bleached to kill mildew, and part of that might be because some of the previous sealer remained. Smoothness is really not the issue and the boards are in excellent condition.

Knock yourself out.