If the difference in price isn't exhorbitant I would go with the brand. There are some studies showing variations in quality. I'm not sure anti-Xa levels are the only thing that matters even though that is the test for anticoagulation.
http://www.theoncologynurse.com/top-issues/2013-issues/august-2013-vol-6-no-3/15955-branded-versus-generic-enoxaparin-biological-activity-may-differ-top