The simplest reason to run the boards width-ways (10-foot lengths) is:

Cleaner Look: You get a perfectly seamless walking surface with no messy staggered joints required along the 32-foot length.

Smarter Installation: It's easier and faster to install, as every board is the same 10-foot length.

Maximum Value: You get highly usable 6-foot cut-offs from your 16-foot Trex boards for your floating extension, saving you material costs.

It is the most practical and visually clean option for your dock.

I hope this helps...