Continental (CRP) makes them on the aftermarket for significantly cheaper than the OE belts. I have not been hearing (nor have I experienced with my 560SEC) good things recently with regard to Conti belts, so I'd probably go with the OE one, which will be around $60-70 through parts.com.
It's important to check the belt tensioner at the same time you change the belt. Tensioners are a common "wear" item,
There are two kinds of belts (different rib counts, early and later motors) so be sure you get the right one for your year of car/engine number.
Cheers,
Gerry