What’s the main difference in these drive chains?

The first one gets you a xdr freehub bur limited range and the second one a hg freehub and a broader range.

I’d say get the first option and put an gx axs derailleur with a 10-52 casette on there for bikepacking.

Mullet refers to having a big cog on a 1x drivetrain. If you look at the descriptions you’ll see that the mullet setup is 51t while the other setup is 44t.

Having a cassette where the biggest cog is 51t is great. You have a really easy gear for climbing. Especially useful if you are carrying a lot of kit.

If you don’t need the easier gear you can go with a larger chainring. Then you can get more speed in the smaller cogs. That doesn’t seem to be an option on that website though, the chainring is 40t.

The disadvantage to a mullet is bigger gaps between gears. Roadies like to have more gears with smaller spaces between them so that they can find the perfect gear for their current situation. Other bikers don’t tend to mind too much.

The mullet setup tends to change the components needed. You can’t just chuck a 51t cog on because the derailleur can’t handle it. So you’ll find that the mullet setup tends to use a mountain bike derailleur. You need to make sure that all your components are compatible but this will be taken care for you since you are buying it as a full build kit.