How they compare
These two come from the same company, FriendFinder, and share much of the same member base, so the choice is mostly about which site you'd rather use.
AdultFriendFinder is the bigger, better-known one. A free account lets you browse, and chat rooms and cams are part of the site, but most features, messaging included, need a paid membership: about $40 for a month, or about $20 a month on a yearly plan. Expect upsells, and note that it says some photos may not show real members.
XMatch is FriendFinder's adult personals site: profiles, search by location and interests, and adult chat. It's free to join and browse, and contacting members needs a paid upgrade. Its pages are busy and ad-heavy, and users report many inactive or fake-looking profiles.