What's better....one ink cartridge that does it all or a separate for black and color?

I am shopping around for a printer. Over the years, I have had Lexmark which are ecomonical and dependable. I am thinking about getting a "3 in 1" model..a printer, scanner, and copier. The one I'm looking at uses one cartridge..supposedly one that produces color AND black. Anyone have one or heard about one? I'm wondering..does it need replacing more often than separate cartridges for color and black would require?