Don't use senna! It will make constipation worse via its rebound constipating effects. Senna also has chemicals in it that are toxic with longer term use. Magnesium citrate is well absorbed, but so is magnesium ascorbate. Magnesium ascorbate is magnesium with vitamin c, so you get both at the same time. It is also buffered.

"Senna has been linked to liver toxicity. There is a report of a 52 year old woman who ingested one liter of senna tea per day for over three years. She developed acute liver failure and kidney impairment requiring intensive care therapy. Long-term use of anthraquinones has been linked to the development of colorectal growths (adenomas) and cancer. "

http://altmedicine.about.com/od/completeazindex/a/senna.htm