Function or filter vs computed attribute with css class name

This could use ng-class it self with an expression.

<td ng-class="{ active: user.active, suspended: user.suspended }">

Or if you are passing the cssClass directly from the API then that would be more easier

<td ng-class="user.cssClass">

Edit

1st Approach

Pros: Will give you desired output :)

Cons

1. In your 1st approach by creating filter you are going to return css class name from it. Theoretically it will work, but if you think technically filter is mostly used on the collection object to filter out data based on filter criteria.
2. Also using attribute with the {{}} wouldn't make sense to evaluate a class value, as angular does provide ng-class which is dedicated to do it.

2nd Approach

It looks pretty good. but in this you need to refactor your code bit. By moving it to utilService & lets use that code for ng-class directive by calling method from controller.

HTML

<td ng-class="cssClassComputationCode()">

Service

app.service('utilService', function(){
    var self = this;
    //below method can be testable by injecting its dependency in testing module
    self.cssClassComputationCode = function(){
        var cssClass = '';
        //here the computation thing will happen 
        cssClass = 'active';
        //some more code
        return cssClass;
    };
});

Controller

app.controller('myCtrl', function($scope, utilService){
     //assigning service method reference to controller scope variable
     $scope.computeCssClass = utilService.cssClassComputationCode;

     //other code here

});