What's the Best Magnesium Supplement?

I know magnesium is really more of a mineral than a vitamin but I hope someone can help anyway.

I've had a long term problem with muscle stifness in my neck and shoulders due to my job using a computer most of the day. I tried lots of things but surprisingly I found magnesium to be quite helpful at relaxing muscles. Unforutnatley I also tend to drink a lot of coffee (which I hope to quite or at least reduce) and since caffeine depletes the body of minerals I imagine my body's magnesium levels are pretty low right now. I've been eating magnesium rich foods for a few days but to more quicky build up magnesium reserves I want to take supplements. Unfortunately where I live in the uk the choice of magnesium supplements is pretty limited and only magnesium oxide and magnseium trisilicate are readily available in stores. Unfortuantely i've read on some web sites that magnesium oxide is *not* a recommended way of taking magnesium, and i'm not sure if magnesium trisilciate is really a proper way to get magnesium. (chemistry lessons were a loooong time ago!) if anyone could clear up the matter i'd appreciate it! :)