Confused about SSD with USB 3.2 Gen 2.2. Will I get a faster transfer rate?

Macs do not support USB 3.2 Gen 2x2 ("up to 20 Gbps"). If you connect a USB 3.2 Gen 2x2 device to a Mac, the speed will fall back to a lower one that the Mac and the device have in common.

On a M3 MacBook Air, I think that would be USB 3.1 Gen 2 speed ("up to 10 Gbps"). (The Technical Specifications seem to be missing this information – a curious omission.)

Aside: It is my understanding that USB4 defines three different "up to 20 Gbps" USB data transfer modes, which are incompatible with each other. One of the two new ones has the marketing name "USB4 20 Gbps". For USB4 host ports (like those on Apple Silicon Macs), USB4 20 Gbps support is required and USB 3.2 Gen 2x2 support is optional. In effect, the USB Implementer's Forum threw USB 3.2 Gen 2x2 under the bus.

I don't think you would see much of a difference (if any) between the two. Your Mac can get up to 40 Gbps but requires Thunderbolt 3 or USB 4 for that. With USB 3.2 I think your Mac will get 10 Gbps at most with either SSD, even though in principle the 2x2 one could achieve 20 Gbps, but I don't think it will get that on your Mac, will instead get 10 Gbps because I don't think it will achieve more with 2x2 on a MacBook Air.

For backups, if using Time Machine, after the first large one, other backups are incremental and usually take a few seconds, or minutes at most. But 10 Gbps is about 1 GB/s, plenty fast, that's 1 TB in just 15 minutes.

By the way, the prices I see online for the X9 and X10 are pretty similar. They cost about the same and in practice, I suspect you will get about the same very fast throughput with either.

For the previous-generation M2 MacBook Air, Apple did call things out:

MacBook Air (M2, 2022) - Technical Specifications - Apple Support

"USB 3.1 Gen 2 (up to 10Gb/s)"

I have no idea why they removed that line from the specifications for the M3 MacBook Airs.