Sig P320 Compact and the P320 xCompact?

Grip looks a little different. Slighty rounded off. More textured area on the XCOMPACT for more grip. If you look at the XCOMPACT, there's a groove for ergonomics for your trigger hand's middle finger. All considered ergonomics upgrades by Sig.

Slightly smaller barrel length (3.6 vs 3.9"). More concealable. They liken it to being a sub-compact length with compact-size grip.

Straight trigger over the curved trigger. I have no experience with this but it's supposed to feel smoother.

Different sights. XCOMPACT's sights supposed to be more visible.

Nitro costs $100 or so less from what I'm seeing online.

p320 x-compact is the subcompact slide and barrel on the compact x-frame. 

It won't be more concealable. The slide is shorter but the grip is bigger and has more heel. If you have bigger hands, the x-frame will likely be more comfortable than the standard medium sized compact frame. 

It has the straight trigger, which you may or may not like. It is not the straight trigger with altered geometry that the x-5 legion has. 

It will not be able to accept 15 round mags with the standard baseplate as it can't accommodate the side tabs. 

the x-compact has a removable sight plate so you can install a micro red dot. 

They have different night sigts. the x-compact comes with only the sig xray up front and a tritium rear. The compact  can come with a full set of siglite tritium sites, or just plain 3-dot steel sights. Depends on whcih SKU you are looking at.