The WebSecurity helper class is designed for use with the ExtendedMembershipProvider class (on which SimpleMembershipProvider amongst others are based). If you based your provider and your site is configured to use an ExtendedMembership based provider, the recommended approach is to use WebSecurity (although it exposes only a subset of what the underlying provider interface is capable of.
If your provider is based on the original .NET MembershipProvider and your site is configured accordingly, you will deal directly with the provider: typically the Membership and Role classes.
This article gives a useful overview of the relationship between the different providers and the ways they are intended to be used: http://weblogs.asp.net/jgalloway/archive/2012/08/29/simplemembership-membership-providers-universal-providers-and-the-new-asp-net-4-5-web-forms-and-asp-net-mvc-4-templates.aspx