Those are two of the most popular websites for Mormon men and women. You should also try Match.com. Make sure you customize your search and select Christian / LDS under the Faith options. Give it a try.
NOTE: Obviously the most singles are in areas with a heavy Mormon population - Utah, California, Idaho, Arizona, etc.
Here's more information about those services.
Featured Mormon Dating Services
LDS Singles - This one is my favorite Mormon dating site.
The design of the website is nothing to brag about, but there are some decent features and a lot of singles.
Use the various search options to find men or women who match your important preferences.
You can communicate with people through private email or instant messenger.
There's an included Matching Profile (optional) which further helps you find good matches.
The Singles Events section will tell you when and where there are planned activities. I saw events held all over the place – the U.S.A, a couple in Canada and Finland.
The website is especially popular in America and Italy.
LDS Mingle - If you don’t live in a real big city or you’re not willing to make long distance connections, you could potentially have a tough time finding enough singles at LDSMingle.com. It doesn't have as many members as the previous site.
Besides that, I like some other things about the website.
It's quick and easy to create your profile.
You can use the comprehensive search menu to filter the list down to the single people that match your preferences.